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/manager.rb

@@ -42,10 +42,11 @@ class Stylesheet::Manager
     cache.clear_regex(/#{plugin}/)
   end
 
-  def self.color_scheme_cache_key(color_scheme, theme_id = nil)
+  def self.color_scheme_cache_key(color_scheme, theme_id = nil, dark: false)
     color_scheme_name = Slug.for(color_scheme.name) + color_scheme&.id.to_s
     theme_string = theme_id ? "_theme#{theme_id}" : ""
-    "#{COLOR_SCHEME_STYLESHEET}_#{color_scheme_name}_#{theme_string}_#{Discourse.current_hostname}"
+    dark_string = dark ? "_dark" : ""
+    "#{COLOR_SCHEME_STYLESHEET}_#{color_scheme_name}_#{theme_string}_#{Discourse.current_hostname}#{dark_string}"
   end
 
   def self.precompile_css
@@ -114,14 +115,17 @@ class Stylesheet::Manager
       theme = manager.get_theme(theme_id)
 
       [theme_color_scheme, *color_schemes].compact.uniq.each do |scheme|
-        $stderr.puts "precompile target: #{COLOR_SCHEME_STYLESHEET} #{theme.name} (#{scheme.name})"
-
-        Stylesheet::Manager::Builder.new(
-          target: COLOR_SCHEME_STYLESHEET,
-          theme: theme,
-          color_scheme: scheme,
-          manager: manager,
-        ).compile(force: true)
+        [true, false].each do |dark|
+          mode = dark ? "dark" : "light"
+          $stderr.puts "precompile target: #{COLOR_SCHEME_STYLESHEET} #{theme.name} (#{scheme.name}) (#{mode})"
+          Stylesheet::Manager::Builder.new(
+            target: COLOR_SCHEME_STYLESHEET,
+            theme: theme,
+            color_scheme: scheme,
+            manager: manager,
+            dark:,
+          ).compile(force: true)
+        end
       end
 
       clear_color_scheme_cache!
@@ -336,7 +340,7 @@ class Stylesheet::Manager
     end
   end
 
-  def color_scheme_stylesheet_details(color_scheme_id = nil, media)
+  def color_scheme_stylesheet_details(color_scheme_id = nil, media, dark: false)
     theme_id = @theme_id || SiteSetting.default_theme_id
 
     color_scheme =
@@ -353,10 +357,10 @@ class Stylesheet::Manager
 
     target = COLOR_SCHEME_STYLESHEET.to_sym
     current_hostname = Discourse.current_hostname
-    cache_key = self.class.color_scheme_cache_key(color_scheme, theme_id)
+    cache_key = self.class.color_scheme_cache_key(color_scheme, theme_id, dark:)
 
     cache.defer_get_set(cache_key) do
-      stylesheet = { color_scheme_id: color_scheme.id }
+      stylesheet = { color_scheme_id: color_scheme.id, dark: }
 
       theme = get_theme(theme_id)
 
@@ -366,6 +370,7 @@ class Stylesheet::Manager
           theme: get_theme(theme_id),
           color_scheme: color_scheme,
           manager: self,
+          dark:,
         )
 
       builder.compile unless File.exist?(builder.stylesheet_fullpath)
@@ -376,8 +381,8 @@ class Stylesheet::Manager
     end
   end
 
-  def color_scheme_stylesheet_preload_tag(color_scheme_id = nil, media = "all")
-    stylesheet = color_scheme_stylesheet_details(color_scheme_id, media)
+  def color_scheme_stylesheet_preload_tag(color_scheme_id = nil, media = "all", dark: false)
+    stylesheet = color_scheme_stylesheet_details(color_scheme_id, media, dark:)
 
     return "" if !stylesheet
 
@@ -386,8 +391,13 @@ class Stylesheet::Manager
     %[<link href="#{href}" rel="preload" as="style"/>].html_safe
   end
 
-  def color_scheme_stylesheet_link_tag(color_scheme_id = nil, media = "all", preload_callback = nil)
-    stylesheet = color_scheme_stylesheet_details(color_scheme_id, media)
+  def color_scheme_stylesheet_link_tag(
+    color_scheme_id = nil,
+    media = "all",
+    preload_callback = nil,
+    dark: false
+  )
+    stylesheet = color_scheme_stylesheet_details(color_scheme_id, media, dark:)
 
     return "" if !stylesheet