Color module doesn't look for CSS files declared in a library

Working on #1813186: Integrate scripts/stylesheets from info/layout files with libraries I found out that color module only look for CSS files declared in the info file with the stylesheets key. It doesn't look for CSS files that are declared in a library. For example see the bartik changes in #1813186-11: Integrate scripts/stylesheets from info/layout files with libraries.