CSSMin expands URLs to images and such when it does path rewriting. It also runs these paths through wfExpandUrl(), which means they will usually end up starting with http:// (even with a protocol-relative $wgServer). We can't use magic request protocol-based switching between http and https either due to caching, so we should just output protocol-relative (or even URLs relative to the root? I think there's a reason we don't do that but I forget) URLs here.
Reported by GreenReaper (CC).