Unskin your Control Panel
I deal with this on almost every CS implementation I work on. We start tweaking the common.css and our control panel goes crazy: our nav buttons get fat, the colors start changing in random ways...
You've probably experienced this if you've ever skinned a CS2.1 or older site. The reason for this is that the control panel master page includes the common.css file. That's fine but how about getting the original look back in the control panel?
To keep from skinning your control panel when you skin your site, you just need to make a copy of your original common.css file and update your controlpanelmaster.ascx file.
Make a copy of an unmodified /themes/default/style/common.css and name it something like cpcommon.css (for control panel, clever, huh?). Drop the copied file in the style directory of your theme. Now edit your controlpanel master file in /themes/[skinname]/masters/ControlPanelMaster.ascx and find the reference to "Common.css" and change the reference to cpcommon.css.
That's it - no more fat nav buttons or funky control panel colors (unless, of course, you're into that sort of thing).