diff --git a/private_dot_config/private_fish/functions/private___tide_apply_terminal_appearance.fish b/private_dot_config/private_fish/functions/private___tide_apply_terminal_appearance.fish
index 5f23781..f92f571 100644
--- a/private_dot_config/private_fish/functions/private___tide_apply_terminal_appearance.fish
+++ b/private_dot_config/private_fish/functions/private___tide_apply_terminal_appearance.fish
@@ -1,8 +1,61 @@
-function __tide_apply_terminal_appearance --argument-names appearance
+function __tide_apply_terminal_appearance --argument-names appearance background
     contains -- $appearance dark light
     or return 2
 
+    # The palette Tide ships is authored for a dark terminal, where every
+    # segment shares this background.
+    set -l dark_segment_background 303030
+    # The background __tide_terminal_light_color's foregrounds are tuned against,
+    # and the ratio they are tuned to.
+    set -l reference_background E4E4E4
+    set -l reference_contrast 4.5
+    # How far the segment background sits from the terminal background. Set
+    # $__tide_terminal_segment_contrast to taste: 1.04:1 is what a fixed E4E4E4
+    # gave on a warm background and is invisible, 1.4:1 reads heavy.
+    set -l segment_contrast 1.12
+    if string match -qr '^[0-9]+(\.[0-9]+)?$' -- "$__tide_terminal_segment_contrast"
+        and test (math -s0 "round($__tide_terminal_segment_contrast * 100)") -gt 100
+        set segment_contrast $__tide_terminal_segment_contrast
+    end
+    # What the foregrounds are re-aimed at. The extra margin over
+    # $reference_contrast absorbs the rounding back to 8 bits per channel.
+    set -l foreground_contrast 4.55
+    # Used when the terminal will not say what its background is. White is the
+    # common default for light terminals.
+    set -l fallback_background FFFFFF
+
+    set -l segment_background
+    set -l foreground_factor 1
+
+    if test "$appearance" = light
+        test -z "$background"
+        and set background (__tide_terminal_background_color light)
+        string match -qr '^[0-9A-Fa-f]{6}$' -- "$background"
+        or set background $fallback_background
+        set background (string upper -- $background)
+
+        # Place the segment a fixed contrast step below the terminal background,
+        # so it picks up that background's tint instead of imposing a grey.
+        set -l background_luminance (math "max(0.000001, "(__tide_relative_luminance $background)")")
+        set -l segment_luminance (__tide_contrast_ceiling $background_luminance $segment_contrast)
+        set segment_background (__tide_scale_luminance $background \
+            (math "$segment_luminance / $background_luminance"))
+
+        # Every light foreground is tuned to sit at $reference_contrast against
+        # the reference background, so the palette shares one luminance ceiling.
+        # A darker segment lowers that ceiling, and moving every foreground by
+        # the same luminance factor walks them all down to it at once.
+        set -l reference_ceiling (__tide_contrast_ceiling \
+            (__tide_relative_luminance $reference_background) $reference_contrast)
+        set -l segment_ceiling (__tide_contrast_ceiling \
+            (__tide_relative_luminance $segment_background) $foreground_contrast)
+        set foreground_factor (math "min(1, $segment_ceiling / $reference_ceiling)")
+    else
+        set background ''
+    end
+
     if test "$__tide_terminal_appearance" = "$appearance"
+        and test "$__tide_terminal_background" = "$background"
         return 1
     end
 
@@ -14,6 +67,14 @@ function __tide_apply_terminal_appearance --argument-names appearance
             string match -q 'tide_*color*' -- $name
             or continue
 
+            # These overrides are exported, so a Fish started from a Fish that
+            # already applied a palette inherits them and would otherwise
+            # capture that palette as its baseline. Dropping the inherited
+            # global makes the name resolve to its universal value again, which
+            # is the configured dark palette. Every name here comes from the
+            # universal scope, so there is always something left to resolve to.
+            set --erase --global $name
+
             set -l current_value $$name
             test (count $current_value) -eq 1
             or continue
@@ -31,11 +92,25 @@ function __tide_apply_terminal_appearance --argument-names appearance
     for index in (seq (count $__tide_terminal_color_names))
         set -l name $__tide_terminal_color_names[$index]
         set -l color $__tide_terminal_dark_colors[$index]
+
         if test "$appearance" = light
-            set color (__tide_terminal_light_color $color)
+            if string match -q '*bg_color*' -- $name
+                and test (string upper -- $color) = $dark_segment_background
+                set color $segment_background
+            else
+                set -l light_color (__tide_terminal_light_color $color)
+                set -l dimmed (__tide_scale_luminance $light_color $foreground_factor)
+                if test -n "$dimmed"
+                    set color $dimmed
+                else
+                    set color $light_color
+                end
+            end
         end
+
         set --global --export $name $color
     end
 
     set --global __tide_terminal_appearance $appearance
+    set --global __tide_terminal_background $background
 end
diff --git a/private_dot_config/private_fish/functions/private___tide_contrast_ceiling.fish b/private_dot_config/private_fish/functions/private___tide_contrast_ceiling.fish
new file mode 100644
index 0000000..27c70e6
--- /dev/null
+++ b/private_dot_config/private_fish/functions/private___tide_contrast_ceiling.fish
@@ -0,0 +1,12 @@
+function __tide_contrast_ceiling --argument-names luminance ratio \
+    --description 'Print the highest luminance that still meets a contrast ratio against a background'
+
+    string match -qr '^[0-9]+(\.[0-9]+)?$' -- $luminance
+    or return 1
+    string match -qr '^[0-9]+(\.[0-9]+)?$' -- $ratio
+    or return 1
+
+    # WCAG contrast is (lighter + 0.05) / (darker + 0.05), so the darkest
+    # partner a background can carry at a given ratio falls straight out of it.
+    math "max(0, ($luminance + 0.05) / $ratio - 0.05)"
+end
diff --git a/private_dot_config/private_fish/functions/private___tide_relative_luminance.fish b/private_dot_config/private_fish/functions/private___tide_relative_luminance.fish
new file mode 100644
index 0000000..09058f2
--- /dev/null
+++ b/private_dot_config/private_fish/functions/private___tide_relative_luminance.fish
@@ -0,0 +1,15 @@
+function __tide_relative_luminance --argument-names hex \
+    --description 'Print the WCAG relative luminance of a six-digit hex color'
+
+    string match -qr '^[0-9A-Fa-f]{6}$' -- $hex
+    or return 1
+
+    set -l r (string sub --start 1 --length 2 -- $hex)
+    set -l g (string sub --start 3 --length 2 -- $hex)
+    set -l b (string sub --start 5 --length 2 -- $hex)
+
+    # sRGB relative luminance. WCAG's piecewise-linear segment for channels
+    # below 0.04045 is left out: it only applies under 10/255 and shifts the
+    # result by less than 0.001, which never changes a palette decision here.
+    math "0.2126 * ((0x$r / 255 + 0.055) / 1.055) ^ 2.4 + 0.7152 * ((0x$g / 255 + 0.055) / 1.055) ^ 2.4 + 0.0722 * ((0x$b / 255 + 0.055) / 1.055) ^ 2.4"
+end
diff --git a/private_dot_config/private_fish/functions/private___tide_scale_luminance.fish b/private_dot_config/private_fish/functions/private___tide_scale_luminance.fish
new file mode 100644
index 0000000..a781ffc
--- /dev/null
+++ b/private_dot_config/private_fish/functions/private___tide_scale_luminance.fish
@@ -0,0 +1,26 @@
+function __tide_scale_luminance --argument-names hex factor \
+    --description 'Multiply a color\'s luminance by a factor, keeping its hue and saturation'
+
+    string match -qr '^[0-9A-Fa-f]{6}$' -- $hex
+    or return 1
+    string match -qr '^[0-9]+(\.[0-9]+)?$' -- $factor
+    or return 1
+
+    set -l channels
+
+    for start in 1 3 5
+        set -l encoded (string sub --start $start --length 2 -- $hex)
+        # Decode to linear light, scale, re-encode. Scaling there multiplies
+        # relative luminance by exactly this factor, because luminance is a
+        # weighted sum of the linear channels. Scaling the encoded channels
+        # instead would not: sRGB's +0.055 offset makes the error grow as the
+        # color gets darker, which is where the contrast floor actually bites.
+        set -l linear "($factor * ((0x$encoded / 255 + 0.055) / 1.055) ^ 2.4)"
+        set --append channels "round(255 * max(0, min(1, (1.055 * $linear ^ (1 / 2.4)) - 0.055)))"
+    end
+
+    # One math call per color rather than three: the bytes are packed into a
+    # single number and printed back out as hex. -s0 truncates, so each
+    # channel does its own rounding above.
+    printf %06X (math -s0 "$channels[1] * 65536 + $channels[2] * 256 + $channels[3]")
+end
diff --git a/private_dot_config/private_fish/functions/private___tide_terminal_background_color.fish b/private_dot_config/private_fish/functions/private___tide_terminal_background_color.fish
new file mode 100644
index 0000000..25c315b
--- /dev/null
+++ b/private_dot_config/private_fish/functions/private___tide_terminal_background_color.fish
@@ -0,0 +1,46 @@
+function __tide_terminal_background_color --argument-names appearance \
+    --description 'Print the background color kitty is showing for an appearance'
+
+    contains -- $appearance dark light
+    or return 1
+
+    functions -q __kitty_config_dir
+    or return 1
+
+    set -l config_dir (__kitty_config_dir)
+
+    # kitty gives the *-theme.auto.conf files priority over includes and even
+    # over --override, so whatever is in them is what the window is actually
+    # showing -- whether it got there from `kitten themes` or from
+    # apply-theme-mode.sh pinning a mode.
+    for file in $config_dir/$appearance-theme.auto.conf $config_dir/no-preference-theme.auto.conf
+        test -r $file
+        or continue
+
+        set -l hex (string match -gr '^\s*background\s+#?([0-9A-Fa-f]{6})\s*$' <$file | tail -n 1)
+        test -n "$hex"
+        or continue
+        set hex (string upper -- $hex)
+
+        set -l luminance (__tide_relative_luminance $hex)
+        test -n "$luminance"
+        or continue
+
+        # A background that disagrees with the appearance Fish reported means
+        # we read the wrong file. A pill derived from the wrong background is
+        # worse than the static fallback, so skip it.
+        set -l scaled (math -s0 "round($luminance * 1000)")
+        if test "$appearance" = light
+            test $scaled -gt 500
+            or continue
+        else
+            test $scaled -lt 500
+            or continue
+        end
+
+        echo $hex
+        return 0
+    end
+
+    return 1
+end
diff --git a/private_dot_config/private_fish/private_docs/private_superpowers/private_specs/private_2026-07-27-tide-terminal-appearance-design.md b/private_dot_config/private_fish/private_docs/private_superpowers/private_specs/private_2026-07-27-tide-terminal-appearance-design.md
new file mode 100644
index 0000000..0b05c22
--- /dev/null
+++ b/private_dot_config/private_fish/private_docs/private_superpowers/private_specs/private_2026-07-27-tide-terminal-appearance-design.md
@@ -0,0 +1,102 @@
+# Tide Terminal Appearance Design
+
+## Goal
+
+Keep the existing Tide prompt unchanged except for its colors. The prompt must
+follow the active Kitty or Ghostty terminal appearance, update while the shell
+is open, and continue to work inside tmux.
+
+## Non-goals
+
+- Do not modify Tide's installed functions.
+- Do not change prompt items, icons, separators, shape, spacing, or layout.
+- Do not infer appearance from macOS.
+- Do not share one terminal's appearance with unrelated Fish sessions.
+
+## Design
+
+Add one Fish integration file under `conf.d/`.
+
+Fish 4.8 already negotiates terminal color-scheme notifications with Kitty,
+Ghostty, and tmux, consumes the terminal replies, queries the actual background,
+and publishes the result as the read-only `fish_terminal_color_theme` variable.
+The integration subscribes to changes of that variable instead of parsing raw
+terminal input itself.
+
+The handler changes only Tide color variables:
+
+- Dark mode keeps the currently configured values exactly.
+- Light mode substitutes darker, contrast-safe equivalents for foreground
+  colors while preserving each color's semantic role, and replaces the shared
+  segment background `303030` with a shade of the terminal's own background.
+
+The light segment background is derived rather than fixed. A single neutral
+value cannot sit at a usable distance from every light theme -- `E4E4E4` is
+1.03:1 against Kaolin Breeze's `EBE8E4`, which is invisible -- and a neutral
+grey reads as a foreign object on a warm or cool background. So the handler
+reads the background kitty is showing and places the segment a fixed contrast
+step below it, which keeps the segment in the theme's own tint.
+
+That step is a judgement call rather than a threshold, and it is not only about
+the segment: the foregrounds are re-aimed at whatever the segment becomes, so a
+heavier step darkens the text along with it and the segment starts reading as a
+blob. `1.12:1` is the default. `__tide_terminal_segment_contrast` overrides it,
+and values that are not a ratio above 1:1 are ignored, since they would put the
+segment level with or above its own background.
+
+kitty's background comes from the `*-theme.auto.conf` files. kitty gives those
+priority over includes and over `--override`, so they are what a window is
+actually showing however the theme got there. A background whose luminance
+contradicts the appearance Fish reported means the wrong file was read, and is
+discarded in favour of the fallback. Terminals that are not kitty get the same
+treatment against an assumed white background.
+
+Moving the segment invalidates the foreground palette, because every light
+foreground is authored to sit at exactly 4.5:1 against the reference background
+`E4E4E4`, leaving no headroom. Since the whole palette shares one luminance
+ceiling, a darker segment lowers that ceiling by a single ratio, and every
+foreground is multiplied by it. That scaling happens in linear light, where
+relative luminance is a weighted sum of the channels and therefore scales
+exactly; scaling the encoded channels instead would drift by more as colors get
+darker, which is where the contrast floor actually binds.
+
+The overrides are exported, so a Fish started from a Fish that already applied a
+palette inherits them. The capture step drops those inherited globals first, so
+each process reads its baseline from the universal scope Tide configures rather
+than from its parent's active palette.
+
+Overrides use exported global scope. This shadows Tide's universal defaults for
+the current Fish process and passes the selected palette to Tide's background
+rendering subprocess without affecting Fish sessions in other terminal windows.
+
+After a scheme event, the handler refreshes Tide's cached colors, reloads the
+generated prompt functions, and requests a command-line repaint. Reapplying the
+current scheme is a no-op.
+
+## Failure behavior
+
+On terminals that do not report a usable appearance,
+`fish_terminal_color_theme` remains `unknown` and Tide keeps its existing dark
+configuration. Fish owns the terminal protocol bytes, so they never become user
+key bindings or command-line input.
+
+## Verification
+
+Automated Fish tests verify:
+
+- only Tide color variables change;
+- non-color Tide configuration remains identical;
+- dark values equal the current configuration;
+- light foregrounds meet the chosen contrast threshold against the derived
+  segment background, on both a theme background and the fallback;
+- scaling a color's luminance moves it by the requested factor, whatever its hue;
+- the background probe reads the active theme and rejects one that contradicts
+  the reported appearance;
+- exported global overrides reach a child Fish process;
+- repeated events are idempotent, and a changed background is not a repeat;
+- the integration subscribes to Fish's native terminal-theme variable;
+- Fish parses every changed file.
+
+Pseudo-terminal acceptance tests emulate light/dark terminal responses and
+cover both direct Fish and Fish through tmux. Kitty and Ghostty share the
+protocol that Fish consumes.
diff --git a/private_dot_config/private_fish/private_tests/private_assert_tide_light_contrast.mjs b/private_dot_config/private_fish/private_tests/private_assert_tide_light_contrast.mjs
index 4e1110d..6593045 100644
--- a/private_dot_config/private_fish/private_tests/private_assert_tide_light_contrast.mjs
+++ b/private_dot_config/private_fish/private_tests/private_assert_tide_light_contrast.mjs
@@ -22,7 +22,13 @@ const luminance = (hex) => {
   );
 };
 
-const backgroundLuminance = luminance('E4E4E4');
+const background = (process.argv[2] ?? 'E4E4E4').toUpperCase();
+if (!/^[0-9A-F]{6}$/.test(background)) {
+  console.error(`background ${background}: expected a six-digit color`);
+  process.exit(1);
+}
+
+const backgroundLuminance = luminance(background);
 const failures = [];
 
 for (const line of input.split('\n')) {
@@ -40,7 +46,9 @@ for (const line of input.split('\n')) {
   );
 
   if (ratio < 4.5) {
-    failures.push(`${dark} -> ${light}: contrast ${ratio.toFixed(2)} is below 4.5`);
+    failures.push(
+      `${dark} -> ${light}: contrast ${ratio.toFixed(2)} against ${background} is below 4.5`,
+    );
   }
 }
 
diff --git a/private_dot_config/private_fish/private_tests/private_fixtures/private_kitty/private_dark-theme.auto.conf b/private_dot_config/private_fish/private_tests/private_fixtures/private_kitty/private_dark-theme.auto.conf
new file mode 100644
index 0000000..7767068
--- /dev/null
+++ b/private_dot_config/private_fish/private_tests/private_fixtures/private_kitty/private_dark-theme.auto.conf
@@ -0,0 +1,3 @@
+# vim:ft=kitty
+background #000000
+foreground #FFFFFF
diff --git a/private_dot_config/private_fish/private_tests/private_fixtures/private_kitty/private_light-theme.auto.conf b/private_dot_config/private_fish/private_tests/private_fixtures/private_kitty/private_light-theme.auto.conf
new file mode 100644
index 0000000..dbbf586
--- /dev/null
+++ b/private_dot_config/private_fish/private_tests/private_fixtures/private_kitty/private_light-theme.auto.conf
@@ -0,0 +1,3 @@
+# vim:ft=kitty
+background #FFFFFF
+foreground #000000
diff --git a/private_dot_config/private_fish/private_tests/private_test_tide_terminal_appearance.fish b/private_dot_config/private_fish/private_tests/private_test_tide_terminal_appearance.fish
index 089329a..8e2e6c2 100644
--- a/private_dot_config/private_fish/private_tests/private_test_tide_terminal_appearance.fish
+++ b/private_dot_config/private_fish/private_tests/private_test_tide_terminal_appearance.fish
@@ -33,17 +33,34 @@ end
 
 set -l light_mapper $project_root/functions/__tide_terminal_light_color.fish
 set -l appearance_applicator $project_root/functions/__tide_apply_terminal_appearance.fish
+set -l luminance_helper $project_root/functions/__tide_relative_luminance.fish
+set -l scale_helper $project_root/functions/__tide_scale_luminance.fish
+set -l ceiling_helper $project_root/functions/__tide_contrast_ceiling.fish
+set -l background_probe $project_root/functions/__tide_terminal_background_color.fish
 
-test -f $light_mapper
-or __test_fail "missing light palette mapper"
-test -f $appearance_applicator
-or __test_fail "missing terminal appearance applicator"
+for file in $light_mapper $appearance_applicator $luminance_helper $scale_helper \
+    $ceiling_helper $background_probe
+    test -f $file
+    or __test_fail "missing "(path basename $file)
+end
 
 source $light_mapper
+source $luminance_helper
+source $scale_helper
+source $ceiling_helper
+source $background_probe
 source $appearance_applicator
 
+# The applicator exports its overrides, so a shell that already applied a
+# palette passes them to this one through the environment. Dropping the
+# inherited globals makes every tide_* name resolve to its universal value
+# again, which is the configured dark palette this suite is written against.
+for name in (set --universal --names | string match -e -r '^tide_')
+    set --erase --global $name
+end
+
 __test_assert_equal E4E4E4 (__tide_terminal_light_color 303030) \
-    "dark segment background maps to the light segment background"
+    "dark segment background maps to the reference light segment background"
 __test_assert_equal 006C9E (__tide_terminal_light_color 00AFFF) \
     "bright blue maps to its contrast-safe light equivalent"
 __test_assert_equal CF0000 (__tide_terminal_light_color FF0000) \
@@ -51,13 +68,61 @@ __test_assert_equal CF0000 (__tide_terminal_light_color FF0000) \
 __test_assert_equal glyph (__tide_terminal_light_color glyph) \
     "non-color values are preserved"
 
+set -l reference_luminance (__tide_relative_luminance E4E4E4)
+test (math -s0 "round($reference_luminance * 1000)") -eq 776
+or __test_fail "relative luminance of E4E4E4: expected 0.776, got $reference_luminance"
+
+# Halving a color's luminance has to actually halve it, whatever its hue.
+# Only the rounding back to 8 bits per channel is allowed to show up.
+for color in EBE8E4 006C9E 7B6400 4C6C6C
+    set -l halved (__tide_scale_luminance $color 0.5)
+    set -l ratio (math (__tide_relative_luminance $halved)" / "(__tide_relative_luminance $color))
+    set -l scaled (math -s0 "round($ratio * 1000)")
+    test $scaled -ge 490 -a $scaled -le 510
+    or __test_fail "scaling $color by 0.5 moved luminance by $ratio, not 0.5"
+end
+
+__test_assert_equal FFFFFF (__tide_scale_luminance 808080 100) \
+    "scaling clamps channels at the top of the range"
+__test_assert_equal 000000 (__tide_scale_luminance 808080 0) \
+    "scaling clamps channels at the bottom of the range"
+__tide_scale_luminance nothex 0.5
+__test_assert_equal 1 $status "scaling rejects values that are not colors"
+
+__test_assert_equal 0.116667 (__tide_contrast_ceiling 0.7 4.5) \
+    "the contrast ceiling is the darkest partner a background can carry"
+__test_assert_equal 0 (__tide_contrast_ceiling 0.1 4.5) \
+    "an unreachable contrast ceiling clamps at zero"
+
+set -l fake_config (mktemp -d)
+printf 'background            #EBE8E4\n' >$fake_config/light-theme.auto.conf
+# A light background under a dark appearance means the wrong file was read, so
+# the probe has to skip it and keep looking.
+printf 'background #FFFFFF\n' >$fake_config/dark-theme.auto.conf
+printf 'background #101010\n' >$fake_config/no-preference-theme.auto.conf
+
+__test_assert_equal EBE8E4 \
+    (KITTY_CONFIG_DIRECTORY=$fake_config __tide_terminal_background_color light) \
+    "the probe reads the background kitty is showing"
+__test_assert_equal 101010 \
+    (KITTY_CONFIG_DIRECTORY=$fake_config __tide_terminal_background_color dark) \
+    "the probe rejects a background that contradicts the appearance"
+
+set -l empty_config (mktemp -d)
+KITTY_CONFIG_DIRECTORY=$empty_config __tide_terminal_background_color light
+__test_assert_equal 1 $status "the probe reports failure when no theme is readable"
+
+rm -rf $fake_config $empty_config
+
 set -l empty_palette_result (
     fish --no-config -c '
-        source $argv[1]
-        source $argv[2]
-        __tide_apply_terminal_appearance light
+        for file in $argv[2..]
+            source $file
+        end
+        __tide_apply_terminal_appearance light $argv[1]
         printf "status=%s" $status
-    ' $light_mapper $appearance_applicator 2>&1 |
+    ' EBE8E4 $light_mapper $luminance_helper $scale_helper $ceiling_helper \
+        $background_probe $appearance_applicator 2>&1 |
         string collect
 )
 __test_assert_equal status=2 $empty_palette_result \
@@ -77,19 +142,19 @@ or __test_fail "the test fixture has no Tide colors"
 
 set -l before_non_colors (string join \x1e (__test_snapshot no))
 
-__tide_apply_terminal_appearance light
+__tide_apply_terminal_appearance light EBE8E4
 __test_assert_equal 0 $status "light appearance applies successfully"
-__test_assert_equal E4E4E4 $tide_pwd_bg_color \
-    "light appearance replaces the existing prompt segment background"
-__test_assert_equal 006C9E $tide_pwd_color_anchors \
-    "light appearance replaces the existing prompt foreground"
+__test_assert_equal DFDCD8 $tide_pwd_bg_color \
+    "light appearance derives the segment background from the terminal background"
+__test_assert_equal 006696 $tide_pwd_color_anchors \
+    "light appearance shades foregrounds down to the derived segment background"
 
 set -l pwd_scope (set --show tide_pwd_bg_color | string collect)
 string match -q '*set in global scope, exported*' -- $pwd_scope
 or __test_fail "Tide overrides are not global and exported"
 
 set -l child_background (fish --no-config -c 'printf %s "$tide_pwd_bg_color"')
-__test_assert_equal E4E4E4 $child_background \
+__test_assert_equal DFDCD8 $child_background \
     "Tide renderer subprocess inherits the light background"
 
 set -l after_non_colors (string join \x1e (__test_snapshot no))
@@ -101,7 +166,7 @@ for name in $expected_color_names
     set --append first_light_state "$name="(string escape -- $$name)
 end
 
-__tide_apply_terminal_appearance light
+__tide_apply_terminal_appearance light EBE8E4
 __test_assert_equal 1 $status "repeated light appearance is a no-op"
 
 set -l second_light_state
@@ -112,14 +177,60 @@ __test_assert_equal (string join \x1e $first_light_state) \
     (string join \x1e $second_light_state) \
     "repeated light appearance preserves all color overrides"
 
-begin
-    for index in (seq (count $expected_dark_colors))
-        set -l dark (string upper -- $expected_dark_colors[$index])
-        test "$dark" = 303030; and continue
-        printf '%s %s\n' $dark (__tide_terminal_light_color $dark)
-    end
-end | node $project_root/tests/assert_tide_light_contrast.mjs
-or __test_fail "light palette foreground contrast is below 4.5:1"
+set --global __test_color_names $expected_color_names
+set --global __test_dark_colors $expected_dark_colors
+set --global __test_contrast_checker $project_root/tests/assert_tide_light_contrast.mjs
+
+function __test_assert_light_contrast --argument-names label
+    begin
+        for index in (seq (count $__test_color_names))
+            set -l name $__test_color_names[$index]
+            set -l dark (string upper -- $__test_dark_colors[$index])
+            test "$dark" = 303030; and continue
+            printf '%s %s\n' $dark (string upper -- $$name)
+        end
+    end | node $__test_contrast_checker $tide_pwd_bg_color
+    or __test_fail "light palette foreground contrast is below 4.5:1 on $label"
+end
+
+__test_assert_light_contrast "a terminal-derived segment background"
+
+# A different terminal background is a different palette, not a repeat.
+__tide_apply_terminal_appearance light nothex
+__test_assert_equal 0 $status "a changed terminal background re-applies the palette"
+__test_assert_equal F2F2F2 $tide_pwd_bg_color \
+    "an unusable terminal background falls back to white"
+__test_assert_light_contrast "the fallback segment background"
+
+__tide_apply_terminal_appearance light FFFFFF
+__test_assert_equal 1 $status \
+    "a background that resolves to the active one is still a no-op"
+
+__tide_apply_terminal_appearance light EBE8E4
+__test_assert_equal 0 $status "returning to the earlier background re-applies"
+__test_assert_equal DFDCD8 $tide_pwd_bg_color \
+    "returning to the earlier background restores its segment background"
+
+set --global __tide_terminal_segment_contrast 1.4
+set --global __tide_terminal_appearance ''
+__tide_apply_terminal_appearance light EBE8E4
+__test_assert_equal C8C6C2 $tide_pwd_bg_color \
+    "a configured segment contrast overrides the default step"
+__test_assert_light_contrast "a configured segment contrast"
+
+# Anything that is not a ratio above 1:1 would make the segment vanish into the
+# background or sit above it, so it is ignored rather than honoured.
+for invalid in 0.8 1 nonsense ''
+    set --global __tide_terminal_segment_contrast $invalid
+    set --global __tide_terminal_appearance ''
+    __tide_apply_terminal_appearance light EBE8E4
+    __test_assert_equal DFDCD8 $tide_pwd_bg_color \
+        "a segment contrast of <$invalid> falls back to the default step"
+end
+set --erase __tide_terminal_segment_contrast
+
+set --global __tide_terminal_appearance ''
+__tide_apply_terminal_appearance light EBE8E4
 
 __tide_apply_terminal_appearance dark
 __test_assert_equal 0 $status "dark appearance applies successfully"
diff --git a/private_dot_config/private_fish/private_tests/private_verify_tide_terminal_events.exp b/private_dot_config/private_fish/private_tests/private_verify_tide_terminal_events.exp
index 698557c..853469a 100644
--- a/private_dot_config/private_fish/private_tests/private_verify_tide_terminal_events.exp
+++ b/private_dot_config/private_fish/private_tests/private_verify_tide_terminal_events.exp
@@ -3,15 +3,21 @@
 set timeout 30
 log_user 0
 
+# The segment background is derived from the theme kitty is showing, so the
+# emulated terminal responses below and the theme files have to agree. These
+# fixtures are the white/black the responses report.
+set kitty_config [file join [file dirname [file normalize [info script]]] fixtures kitty]
+
 set path_label "direct"
 set through_tmux 0
 if {[llength $argv] > 0 && [lindex $argv 0] eq "tmux"} {
     set path_label "tmux"
     set through_tmux 1
-    spawn env TERM=xterm-kitty tmux -L tide-appearance-test \
+    spawn env TERM=xterm-kitty KITTY_CONFIG_DIRECTORY=$kitty_config \
+        tmux -L tide-appearance-test \
         -f "$env(HOME)/.tmux.conf" new-session fish --interactive
 } else {
-    spawn env TERM=xterm-kitty fish --interactive
+    spawn env TERM=xterm-kitty KITTY_CONFIG_DIRECTORY=$kitty_config fish --interactive
 }
 
 if {$through_tmux} {
@@ -60,7 +66,7 @@ if {$through_tmux} {
 
 send -- "printf 'CHECK:%s:%s\\n' \$__tide_terminal_appearance \$tide_pwd_bg_color\r"
 expect {
-    -re {CHECK:light:E4E4E4} {}
+    -re {CHECK:light:F2F2F2} {}
     timeout {
         puts stderr "The initial light terminal background did not apply the light Tide palette"
         exit 1