DEV: Let themes extend color definitions (#10429)

Themes can now declare custom colors that get compiled in core's color definitions stylesheet, thus allowing themes to better support dark/light color schemes. 

For example, if you need your theme to use tertiary for an element in a light color scheme and quaternary in a dark scheme, you can add the following SCSS to your theme's `color_definitions.scss` file: 

```
:root {
  --mytheme-tertiary-or-quaternary: #{dark-light-choose($tertiary, $quaternary)};
}
```

And then use the `--mytheme-tertiary-or-quaternary` variable as the color property of that element. You can also use this file to add color variables that use SCSS color transformation functions (lighten, darken, saturate, etc.) without compromising your theme's compatibility with different color schemes.

lib/stylesheet/manager.rb

@@ -23,6 +23,10 @@ class Stylesheet::Manager
     cache.hash.keys.select { |k| k =~ /theme/ }.each { |k| cache.delete(k) }
   end
 
+  def self.clear_color_scheme_cache!
+    cache.hash.keys.select { |k| k =~ /color_definitions/ }.each { |k| cache.delete(k) }
+  end
+
   def self.clear_core_cache!(targets)
     cache.hash.keys.select { |k| k =~ /#{targets.join('|')}/ }.each { |k| cache.delete(k) }
   end
@@ -94,13 +98,14 @@ class Stylesheet::Manager
   end
 
   def self.color_scheme_stylesheet_details(color_scheme_id = nil, media, theme_id)
+    theme_id = theme_id || SiteSetting.default_theme_id
+
     color_scheme = begin
       ColorScheme.find(color_scheme_id)
     rescue
       # don't load fallback when requesting dark color scheme
       return false if media != "all"
 
-      theme_id = theme_id || SiteSetting.default_theme_id
       Theme.find_by_id(theme_id)&.color_scheme || ColorScheme.base
     end
 
@@ -108,13 +113,14 @@ class Stylesheet::Manager
 
     target = COLOR_SCHEME_STYLESHEET.to_sym
     current_hostname = Discourse.current_hostname
-    cache_key = color_scheme_cache_key(color_scheme)
+    cache_key = color_scheme_cache_key(color_scheme, theme_id)
     stylesheets = cache[cache_key]
     return stylesheets if stylesheets.present?
 
     stylesheet = { color_scheme_id: color_scheme&.id }
 
-    builder = self.new(target, nil, color_scheme)
+    builder = self.new(target, theme_id, color_scheme)
+
     builder.compile unless File.exists?(builder.stylesheet_fullpath)
 
     href = builder.stylesheet_path(current_hostname)
@@ -132,20 +138,16 @@ class Stylesheet::Manager
     %[<link href="#{href}" media="#{media}" rel="stylesheet"/>].html_safe
   end
 
-  def self.color_scheme_cache_key(color_scheme)
+  def self.color_scheme_cache_key(color_scheme, theme_id = nil)
     color_scheme_name = Slug.for(color_scheme.name) + color_scheme&.id.to_s
-    "#{COLOR_SCHEME_STYLESHEET}_#{color_scheme_name}_#{Discourse.current_hostname}"
-  end
-
-  def self.color_scheme_cache_clear(color_scheme)
-    cache_key = color_scheme_cache_key(color_scheme)
-    cache[cache_key] = nil
+    theme_string = theme_id ? "_theme#{theme_id}" : ""
+    "#{COLOR_SCHEME_STYLESHEET}_#{color_scheme_name}#{theme_string}_#{Discourse.current_hostname}"
   end
 
   def self.precompile_css
-    themes = Theme.where('user_selectable OR id = ?', SiteSetting.default_theme_id).pluck(:id, :name)
+    themes = Theme.where('user_selectable OR id = ?', SiteSetting.default_theme_id).pluck(:id, :name, :color_scheme_id)
     themes << nil
-    themes.each do |id, name|
+    themes.each do |id, name, color_scheme_id|
       [:desktop, :mobile, :desktop_rtl, :mobile_rtl, :desktop_theme, :mobile_theme, :admin].each do |target|
         theme_id = id || SiteSetting.default_theme_id
         next if target =~ THEME_REGEX && theme_id == -1
@@ -156,17 +158,13 @@ class Stylesheet::Manager
         builder.compile(force: true)
         cache[cache_key] = nil
       end
-    end
 
-    cs_ids = Theme.where('user_selectable OR id = ?', SiteSetting.default_theme_id).pluck(:color_scheme_id)
-    ColorScheme.where(id: cs_ids).each do |cs|
-      target = COLOR_SCHEME_STYLESHEET
-      STDERR.puts "precompile target: #{target} #{cs.name}"
+      scheme = ColorScheme.find_by_id(color_scheme_id) || ColorScheme.base
+      STDERR.puts "precompile target: #{COLOR_SCHEME_STYLESHEET} #{name} (#{scheme.name})"
 
-      builder = self.new(target, nil, cs)
+      builder = self.new(COLOR_SCHEME_STYLESHEET, id, scheme)
       builder.compile(force: true)
-      cache_key = color_scheme_cache_key(cs)
-      cache[cache_key] = nil
+      clear_color_scheme_cache!
     end
 
     nil
@@ -349,8 +347,6 @@ class Stylesheet::Manager
   end
 
   def theme_digest
-    scss = ""
-
     if [:mobile_theme, :desktop_theme].include?(@target)
       scss_digest = theme.resolve_baked_field(:common, :scss)
       scss_digest += theme.resolve_baked_field(@target.to_s.sub("_theme", ""), :scss)
@@ -401,7 +397,8 @@ class Stylesheet::Manager
     category_updated = Category.where("uploaded_background_id IS NOT NULL").pluck(:updated_at).map(&:to_i).sum
 
     if cs || category_updated > 0
-      Digest::SHA1.hexdigest "#{RailsMultisite::ConnectionManagement.current_db}-#{cs&.id}-#{cs&.version}-#{Stylesheet::Manager.last_file_updated}-#{category_updated}"
+      theme_color_defs = theme&.resolve_baked_field(:common, :color_definitions)
+      Digest::SHA1.hexdigest "#{RailsMultisite::ConnectionManagement.current_db}-#{cs&.id}-#{cs&.version}-#{theme_color_defs}-#{Stylesheet::Manager.last_file_updated}-#{category_updated}"
     else
       digest_string = "defaults-#{Stylesheet::Manager.last_file_updated}"