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/helpers/application_helper_spec.rb

@@ -877,12 +877,20 @@ RSpec.describe ApplicationHelper do
   describe "#discourse_theme_color_meta_tags" do
     before do
       light = Fabricate(:color_scheme)
-      light.color_scheme_colors << ColorSchemeColor.new(name: "header_background", hex: "abcdef")
+      light.color_scheme_colors << ColorSchemeColor.new(
+        name: "header_background",
+        hex: "abcdef",
+        dark_hex: "fedcba",
+      )
       light.save!
       helper.request.cookies["color_scheme_id"] = light.id
 
       dark = Fabricate(:color_scheme)
-      dark.color_scheme_colors << ColorSchemeColor.new(name: "header_background", hex: "defabc")
+      dark.color_scheme_colors << ColorSchemeColor.new(
+        name: "header_background",
+        hex: "defabc",
+        dark_hex: "cbafed",
+      )
       dark.save!
       helper.request.cookies["dark_scheme_id"] = dark.id
     end
@@ -902,6 +910,17 @@ RSpec.describe ApplicationHelper do
         <meta name="theme-color" media="all" content="#abcdef">
       HTML
     end
+
+    context "when use_overhauled_theme_color_palette setting is true" do
+      before { SiteSetting.use_overhauled_theme_color_palette = true }
+
+      it "renders a light and dark theme-color meta tag using the light and dark palettes of the same color scheme record" do
+        expect(helper.discourse_theme_color_meta_tags).to eq(<<~HTML)
+          <meta name="theme-color" media="(prefers-color-scheme: light)" content="#abcdef">
+          <meta name="theme-color" media="(prefers-color-scheme: dark)" content="#fedcba">
+        HTML
+      end
+    end
   end
 
   describe "#discourse_color_scheme_meta_tag" do
@@ -944,4 +963,27 @@ RSpec.describe ApplicationHelper do
       HTML
     end
   end
+
+  describe "#dark_scheme_id" do
+    fab!(:dark_scheme) { Fabricate(:color_scheme) }
+    fab!(:light_scheme) { Fabricate(:color_scheme) }
+
+    before do
+      helper.request.cookies["color_scheme_id"] = light_scheme.id
+      helper.request.cookies["dark_scheme_id"] = dark_scheme.id
+    end
+
+    it "returns the value set in the dark_scheme_id cookie" do
+      expect(helper.dark_scheme_id).to eq(dark_scheme.id)
+    end
+
+    context "when use_overhauled_theme_color_palette is true" do
+      before { SiteSetting.use_overhauled_theme_color_palette = true }
+
+      it "returns the same value as #scheme_id" do
+        expect(helper.dark_scheme_id).to eq(helper.scheme_id)
+        expect(helper.scheme_id).to eq(light_scheme.id)
+      end
+    end
+  end
 end