FEATURE: Allow themes to override color transformation variables (#7987)

Theme developers can now add any of the transformed color variables to their color scheme in about.json. For example

```
  "color_schemes": {
    "Light": {
      "primary": "333333",
      "secondary": "ffffff",
      "primary-low": "ff0000"
    }
  },
```

would override the primary-low variable when compiling SCSS for the color scheme. The primary-low variable will also be visible in administrator color palette UI.

app/models/theme.rb

@@ -34,20 +34,18 @@ class Theme < ActiveRecord::Base
     where('user_selectable OR id = ?', SiteSetting.default_theme_id)
   }
 
-  def notify_color_change(color)
-    changed_colors << color
+  def notify_color_change(color, scheme: nil)
+    scheme ||= color.color_scheme
+    changed_colors << color if color
+    changed_schemes << scheme if scheme
   end
 
   after_save do
-    color_schemes = {}
-    changed_colors.each do |color|
-      color.save!
-      color_schemes[color.color_scheme_id] ||= color.color_scheme
-    end
-
-    color_schemes.values.each(&:save!)
+    changed_colors.each(&:save!)
+    changed_schemes.each(&:save!)
 
     changed_colors.clear
+    changed_schemes.clear
 
     changed_fields.each(&:save!)
     changed_fields.clear
@@ -343,6 +341,10 @@ class Theme < ActiveRecord::Base
     @changed_colors ||= []
   end
 
+  def changed_schemes
+    @changed_schemes ||= Set.new
+  end
+
   def set_field(target:, name:, value: nil, type: nil, type_id: nil, upload_id: nil)
     name = name.to_s