I just installed a fresh copy of 3.0 and my default admin theme looked fine until a few clicks around the menu and the visual appearance of the default admin theme went wonky. It's still laid out the same, but all of the colors and images are gone and the links are default blue and purple.
I tried every solution in this post to no avail.
The blue admin theme looks fine.
The only odd thing I did after the install was delete the original admin user (there was a typo in the name). Could that royally screw things up?