But I was having the same problem, with the panel icon not showing up. After a bunch of debugging, I found out it was related to this bug:

Bug 30442 - There's extra semi-colon in com_zimbra_local/local.css causing problems

The work around until the bug is fixed is to put a dummy css class as the first line of your css file (or, of course, remove the semi-colon from the local.css file).