`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.
