Custom CSS files do not include cache buster

CSS and Javascript files will include a cache busting string appended to the query string. However, custom CSS files will not, which leads to the case where a new release will fetch the latest CSS files for all others except the custom CSS file.

The appending of the cache buster string to the CSS URL could also be improved since it always appends ?'r=<cache-buster>' regardless of whether the URL already has a query string or not.

This could be solved by improving the URL generation and also applying it to CSS files.