|Issue tags:||needs backport to D7|
The current documentation only indicates that relative paths to css files are supported, however it appears that stream wrapper URIs work fine as well (e.g. public://css_injector/css_injector_1.css).
Support was added here: #927406: Colorable themes fail when CSS aggregation is enabled due to improper stream wrapper usage
A css_injector issue that uses a stream wrapper URI: #1044670: Fix file handling resulting in Incorrect path to css file on Windows