DEV: Backend support for light/dark mode in color palettes (#30893)

We're embarking on a project for overhauling the color palette and theme
systems in Discourse. As part of this project, we're making each color
palette include light and dark modes instead of the status quo of
requiring 2 separate color palettes to implement light and dark modes.

This commit is a first step towards that goal; it adds a code path for
generating and serving `color_definitions` stylesheets using the
built-in dark variant of a color palette. All of this code path is
behind a default-off site setting `use_overhauled_theme_color_palette`,
so there's no change in behavior unless the setting is enabled.

Internal topic: t/141467.

lib/stylesheet/importer.rb

@@ -145,19 +145,21 @@ module Stylesheet
       if @color_scheme_id
         colors =
           begin
-            ColorScheme.find(@color_scheme_id).resolved_colors
+            ColorScheme.find(@color_scheme_id).resolved_colors(dark: @dark)
           rescue StandardError
             ColorScheme.base_colors
           end
       elsif (@theme_id && !theme.component)
-        colors = theme&.color_scheme&.resolved_colors || ColorScheme.base_colors
+        colors = theme&.color_scheme&.resolved_colors(dark: @dark) || ColorScheme.base_colors
       else
         # this is a slightly ugly backwards compatibility fix,
         # we shouldn't be using the default theme color scheme for components
         # (most components use CSS custom properties which work fine without this)
         colors =
-          Theme.find_by_id(SiteSetting.default_theme_id)&.color_scheme&.resolved_colors ||
-            ColorScheme.base_colors
+          Theme
+            .find_by_id(SiteSetting.default_theme_id)
+            &.color_scheme
+            &.resolved_colors(dark: @dark) || ColorScheme.base_colors
       end
 
       colors.each { |n, hex| contents << "$#{n}: ##{hex} !default; " }
@@ -178,6 +180,7 @@ module Stylesheet
       @theme = options[:theme]
       @theme_id = options[:theme_id]
       @color_scheme_id = options[:color_scheme_id]
+      @dark = options[:dark]
 
       if @theme && !@theme_id
         # make up an id so other stuff does not bail out