fish: add theme command to toggle kitty light/dark

`theme` switches kitty between the existing dark-theme.auto.conf and
light-theme.auto.conf via `kitty @ set-colors --all --configured`, so it
stays in sync with whatever `kitten themes --light/--dark` selects.
Subcommands: light, dark, auto (hand control back to the OS appearance
via load-config), status.

Tide is resynced by calling __tide_apply_terminal_appearance directly,
so the prompt recolors even if kitty sends no palette-change
notification for a dynamic color change. conf.d/kitty_theme.fish tracks
$fish_terminal_color_theme into $kitty_theme_mode so the toggle still
reads the real state after kitty follows an OS appearance change.

light-theme.auto.conf had no tab colors, which left the dark tab bar on
a light background after switching; add a matching tab style block.

private_dot_config/kitty/private_light-theme.auto.conf

@@ -20,4 +20,12 @@ color7 #bbbbbb
 color15 #ffffff
 selection_foreground #2a2b33
 
+# START_AUTOGENERATED_TAB_STYLE
+# Feel free to update these colors manually and remove these comments.
+active_tab_foreground   #f8f8f8
+active_tab_background   #2f5af3
+inactive_tab_foreground #2f5af3
+inactive_tab_background #f8f8f8
+# END_AUTOGENERATED_TAB_STYLE
+
 macos_titlebar_color background

private_dot_config/private_fish/conf.d/kitty_theme.fish

@@ -0,0 +1,16 @@
+status is-interactive
+or exit
+
+# Keep $kitty_theme_mode tracking what the terminal actually reports, so that
+# `theme` toggles from the real state even after kitty follows an OS appearance
+# change on its own. fish populates $fish_terminal_color_theme at the first
+# prompt and again whenever the terminal signals a palette change.
+function __kitty_theme_track --on-variable fish_terminal_color_theme
+    contains -- "$fish_terminal_color_theme" light dark
+    or return
+
+    test "$kitty_theme_mode" = "$fish_terminal_color_theme"
+    and return
+
+    set --universal kitty_theme_mode $fish_terminal_color_theme
+end

private_dot_config/private_fish/functions/__kitty_theme_current.fish

@@ -0,0 +1,14 @@
+function __kitty_theme_current --description 'Report the color mode kitty is currently using'
+    # $kitty_theme_mode is kept in sync with the terminal by conf.d/kitty_theme.fish
+    # and by `theme` itself; the rest are fallbacks for shells that have not yet
+    # seen an interactive prompt.
+    if contains -- "$kitty_theme_mode" light dark
+        echo $kitty_theme_mode
+    else if contains -- "$fish_terminal_color_theme" light dark
+        echo $fish_terminal_color_theme
+    else if test "$(defaults read -g AppleInterfaceStyle 2>/dev/null)" = Dark
+        echo dark
+    else
+        echo light
+    end
+end

private_dot_config/private_fish/functions/theme.fish

@@ -0,0 +1,91 @@
+function theme --description 'Switch kitty between its light and dark color themes'
+    set -l config_dir $HOME/.config/kitty
+    set -q KITTY_CONFIG_DIRECTORY
+    and set config_dir $KITTY_CONFIG_DIRECTORY
+
+    if test (count $argv) -gt 1
+        echo "theme: expected at most one argument" >&2
+        return 2
+    end
+
+    set -l target $argv[1]
+    test -z "$target"
+    and set target toggle
+
+    switch $target
+        case -h --help help
+            printf '%s\n' \
+                'Usage: theme [light|dark|auto|status]' \
+                '' \
+                '  theme         toggle between light and dark' \
+                '  theme light   switch to light' \
+                '  theme dark    switch to dark' \
+                '  theme auto    follow the macOS appearance again' \
+                '  theme status  print the current mode'
+            return 0
+        case status
+            __kitty_theme_current
+            return 0
+        case toggle
+            if test (__kitty_theme_current) = dark
+                set target light
+            else
+                set target dark
+            end
+        case auto light dark
+            # handled below
+        case '*'
+            echo "theme: unknown mode '$target' (expected light, dark, auto or status)" >&2
+            return 2
+    end
+
+    if not set -q KITTY_WINDOW_ID
+        echo "theme: not running inside kitty" >&2
+        return 1
+    end
+
+    if test "$target" = auto
+        # Re-reading the config restores kitty's own OS-appearance handling,
+        # which a manual override suppresses until the OS appearance changes.
+        kitty @ load-config
+        or return $status
+
+        if test "$(defaults read -g AppleInterfaceStyle 2>/dev/null)" = Dark
+            set target dark
+        else
+            set target light
+        end
+    else
+        set -l theme_file $config_dir/$target-theme.auto.conf
+        if not test -r $theme_file
+            echo "theme: cannot read $theme_file" >&2
+            return 1
+        end
+
+        # Only pass through settings whose value is a hex color; kitty's
+        # set-colors rejects everything else these files may contain.
+        set -l colors (awk 'NF == 2 && $1 !~ /^#/ && $2 ~ /^#[0-9a-fA-F]+$/ { print $1 "=" $2 }' $theme_file)
+        if test (count $colors) -eq 0
+            echo "theme: no colors found in $theme_file" >&2
+            return 1
+        end
+
+        kitty @ set-colors --all --configured $colors
+        or return $status
+    end
+
+    set --universal kitty_theme_mode $target
+
+    if functions -q __tide_apply_terminal_appearance
+        __tide_apply_terminal_appearance $target
+        set -l apply_status $status
+        if test $apply_status -eq 0
+            functions -q _tide_cache_variables
+            and _tide_cache_variables
+            functions -q _tide_sub_reload
+            and tide reload
+        end
+    end
+
+    echo $target
+end