Hi
I downloaded and examined your skin, then installed it on my dev box, and initially had no problems with it. However, when I tried to edit a text/html module in FreeTextBox, I indeed got an error. This was using your skin set as the "admin" skin. I modified it to remove all panes except the ContentPane, then removed the flash header and got a good error message from DNN.
The problem is with your .css file, but I'm not sure exactly where. It's probably within the first 7 or 8 classes, I see one duplicated there ('.menu'). Anyway, I swapped out the .css file with a known good one (my own!) and your skin worked fine then.
Hope this helps...