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.

app/models/color_scheme.rb

@@ -409,16 +409,18 @@ class ColorScheme < ActiveRecord::Base
     new_color_scheme
   end
 
-  def self.lookup_hex_for_name(name, scheme_id = nil)
+  def self.lookup_hex_for_name(name, scheme_id = nil, dark: false)
     enabled_color_scheme = find_by(id: scheme_id) if scheme_id
     enabled_color_scheme ||= Theme.where(id: SiteSetting.default_theme_id).first&.color_scheme
-    (enabled_color_scheme || base).colors.find { |c| c.name == name }.try(:hex)
+    color_record = (enabled_color_scheme || base).colors.find { |c| c.name == name }
+    return if !color_record
+    dark ? color_record.dark_hex || color_record.hex : color_record.hex
   end
 
-  def self.hex_for_name(name, scheme_id = nil)
-    hex_cache.defer_get_set(scheme_id ? name + "_#{scheme_id}" : name) do
-      lookup_hex_for_name(name, scheme_id)
-    end
+  def self.hex_for_name(name, scheme_id = nil, dark: false)
+    cache_key = scheme_id ? "#{name}_#{scheme_id}" : name
+    cache_key += "_dark" if dark
+    hex_cache.defer_get_set(cache_key) { lookup_hex_for_name(name, scheme_id, dark:) }
   end
 
   def colors=(arr)
@@ -450,10 +452,16 @@ class ColorScheme < ActiveRecord::Base
     colors || ColorScheme.base_colors
   end
 
-  def resolved_colors
+  def resolved_colors(dark: false)
     from_base = ColorScheme.base_colors
     from_custom_scheme = base_colors
-    from_db = colors.map { |c| [c.name, c.hex] }.to_h
+    from_db =
+      colors
+        .map do |c|
+          hex = dark ? (c.dark_hex || c.hex) : c.hex
+          [c.name, hex]
+        end
+        .to_h
 
     resolved = from_base.merge(from_custom_scheme).except("hover", "selected").merge(from_db)
 

app/models/color_scheme_color.rb

@@ -4,6 +4,7 @@ class ColorSchemeColor < ActiveRecord::Base
   belongs_to :color_scheme
 
   validates :hex, format: { with: /\A([0-9a-fA-F]{3}|[0-9a-fA-F]{6})\z/ }
+  validates :dark_hex, format: { with: /\A([0-9a-fA-F]{3}|[0-9a-fA-F]{6})\z/ }, allow_nil: true
 
   def hex_with_hash
     "##{hex}"
@@ -20,6 +21,7 @@ end
 #  color_scheme_id :integer          not null
 #  created_at      :datetime         not null
 #  updated_at      :datetime         not null
+#  dark_hex        :string(6)
 #
 # Indexes
 #