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.

spec/lib/stylesheet/manager_spec.rb

@@ -676,6 +676,73 @@ RSpec.describe Stylesheet::Manager do
       expect(link).to include("/stylesheets/color_definitions_funky-bunch_#{cs.id}_")
     end
 
+    it "generates the dark mode of a color scheme when the dark option is specified" do
+      scheme = ColorScheme.create_from_base(name: "Neutral", base_scheme_id: "Neutral")
+      ColorSchemeRevisor.revise(
+        scheme,
+        colors: [{ name: "primary", hex: "CABFAF", dark_hex: "FAFCAB" }],
+      )
+      theme = Fabricate(:theme)
+      manager = manager(theme.id)
+
+      dark_stylesheet =
+        Stylesheet::Manager::Builder.new(
+          target: :color_definitions,
+          theme: theme,
+          color_scheme: scheme,
+          manager: manager,
+          dark: true,
+        ).compile
+      light_stylesheet =
+        Stylesheet::Manager::Builder.new(
+          target: :color_definitions,
+          theme: theme,
+          color_scheme: scheme,
+          manager: manager,
+        ).compile
+
+      expect(light_stylesheet).to include("--primary: #CABFAF;")
+      expect(light_stylesheet).to include("color_definitions_neutral_#{scheme.id}_#{theme.id}")
+      expect(light_stylesheet).not_to include(
+        "color_definitions_neutral_#{scheme.id}_#{theme.id}_dark",
+      )
+
+      expect(dark_stylesheet).to include("--primary: #FAFCAB;")
+      expect(dark_stylesheet).to include("color_definitions_neutral_#{scheme.id}_#{theme.id}_dark")
+    end
+
+    it "uses the light colors as fallback if the dark scheme doesn't define them" do
+      scheme = ColorScheme.create_from_base(name: "Neutral", base_scheme_id: "Neutral")
+      ColorSchemeRevisor.revise(scheme, colors: [{ name: "primary", hex: "BACFAB", dark_hex: nil }])
+      theme = Fabricate(:theme)
+      manager = manager(theme.id)
+
+      dark_stylesheet =
+        Stylesheet::Manager::Builder.new(
+          target: :color_definitions,
+          theme: theme,
+          color_scheme: scheme,
+          manager: manager,
+          dark: true,
+        ).compile
+      light_stylesheet =
+        Stylesheet::Manager::Builder.new(
+          target: :color_definitions,
+          theme: theme,
+          color_scheme: scheme,
+          manager: manager,
+        ).compile
+
+      expect(light_stylesheet).to include("--primary: #BACFAB;")
+      expect(light_stylesheet).to include("color_definitions_neutral_#{scheme.id}_#{theme.id}")
+      expect(light_stylesheet).not_to include(
+        "color_definitions_neutral_#{scheme.id}_#{theme.id}_dark",
+      )
+
+      expect(dark_stylesheet).to include("--primary: #BACFAB;")
+      expect(dark_stylesheet).to include("color_definitions_neutral_#{scheme.id}_#{theme.id}_dark")
+    end
+
     it "updates outputted colors when updating a color scheme" do
       scheme = ColorScheme.create_from_base(name: "Neutral", base_scheme_id: "Neutral")
       theme = Fabricate(:theme)
@@ -905,7 +972,7 @@ RSpec.describe Stylesheet::Manager do
 
       # Ensure we force compile each theme only once
       expect(output.scan(/#{child_theme_with_css.name}/).length).to eq(2)
-      expect(StylesheetCache.count).to eq(22) # (3 themes * 2 targets) + 16 color schemes (2 themes * 8 color schemes (7 defaults + 1 theme scheme))
+      expect(StylesheetCache.count).to eq(38) # (3 themes * 2 targets) + 32 color schemes (2 themes * 8 color schemes (7 defaults + 1 theme scheme) * 2 (light and dark mode per scheme))
     end
 
     it "generates precompiled CSS - core and themes" do
@@ -913,7 +980,7 @@ RSpec.describe Stylesheet::Manager do
       Stylesheet::Manager.precompile_theme_css
 
       results = StylesheetCache.pluck(:target)
-      expect(results.size).to eq(30) # 11 core targets + 9 theme + 10 color schemes
+      expect(results.size).to eq(46) # 8 core targets + 6 theme + 32 color schemes (light and dark mode per scheme)
 
       theme_targets.each do |tar|
         expect(
@@ -929,7 +996,7 @@ RSpec.describe Stylesheet::Manager do
       Stylesheet::Manager.precompile_theme_css
 
       results = StylesheetCache.pluck(:target)
-      expect(results.size).to eq(30) # 11 core targets + 9 theme + 10 color schemes
+      expect(results.size).to eq(46) # 8 core targets + 6 theme + 32 color schemes (light and dark mode per scheme)
 
       expect(results).to include("color_definitions_#{scheme1.name}_#{scheme1.id}_#{user_theme.id}")
       expect(results).to include(