Until recently I was using Chirpy VS plugin for mashing css files. But I have found it doesn't change css url relative paths during mashing and it is a problem for me now (as I mash css files which access local images but are placed in different directories).
Is there any tool which mashes css files and changes urls relative paths accordingly?
You should check into the Yahoo tool for JS / CSS compression. There is a codeplex project which has Visual Studio post build support. I have tested it explicitly for the scenario you're asking about (relative paths) but it's worth exploring.
I have found this workitem which proves there is a bug in Chirpy.
Also it looks like CrusherModule will solve my problem.
Update. Finally I used CrushIt