FEATURE: Multiple SCSS file support for themes (#7351)

Theme developers can include any number of scss files within the /scss/ directory of a theme. These can then be imported from the main common/desktop/mobile scss.

lib/stylesheet/compiler.rb

@@ -9,7 +9,7 @@ module Stylesheet
     def self.compile_asset(asset, options = {})
 
       if Importer.special_imports[asset.to_s]
-        filename = "theme.scss"
+        filename = "theme_#{options[:theme_id]}.scss"
         file = "@import \"common/foundation/variables\"; @import \"theme_variables\"; @import \"#{asset}\";"
       else
         filename = "#{asset}.scss"