Someone else reported the same problem by email and sent a link to the site. If you view the source of your rendered page then grab the complete css url, it should be like ...csshandler.ashx?.....
Paste the whole url into your browser and instead of seeing css as you should, I expect you will see an error, probably this one:
System.DllNotFoundException: libMonoPosixHelper.so
at (wrapper managed-to-native)
System.IO.Compression.
DeflateStream:create_z_stream
(System.IO.Compression.CompressionMode,bool)
at System.IO.Compression.DeflateStream..ctor (System.IO.Stream
compressedStream, CompressionMode mode, Boolean leaveOpen, Boolean
gzip) [0x00000]
at (wrapper remoting-invoke-with-check)
System.IO.Compression.DeflateStream:.ctor
(System.IO.Stream,System.IO.Compression.CompressionMode,bool,bool)
at System.IO.Compression.GZipStream..ctor (System.IO.Stream
compressedStream, CompressionMode mode, Boolean leaveOpen) [0x00000]
at System.IO.Compression.GZipStream..ctor (System.IO.Stream
compressedStream, CompressionMode mode) [0x00000]
at (wrapper remoting-invoke-with-check)
System.IO.Compression.GZipStream:.ctor
(System.IO.Stream,System.IO.Compression.CompressionMode)
at mojoPortal.Web.UI.CssHandler.ProcessRequest
(System.Web.HttpContext context) [0x00000]
at System.Web.HttpApplication+<Pipeline>c__Iterator2.MoveNext () [0x00000]
at System.Web.HttpApplication.Tick () [0x00000]
As a workaround for this apparent Mono bug, you can try change this from true to false in Web.config (or user.config)
<add key="CombineCSS" value="true" />
This should bypass the css handler and just render urls to static css files. Hopefully that will work for you at least until the Mono bug is fixed.
Best,
Joe