Hide the google bar at the top of gmail. On mouseover of the globe it expands back to the original bar. The globe (hard to see) is from the tango icon set. Other gmail styles I've made can be found at Gmail: sidebar/fixed height and Gmail: MonoSpace non-html emails.

Update 5/10/07:
After an indeterminate amount of time the bar structure in gmail changes. This should fix that problem.

To change the font colors for the app bar:
You should be able to use the selectors that are here (#cb,#cornerBookmarks, etc) and use the descendant selector for the links
(i.e. #cb a { color: red !important; } )

To change the image:
1. Find your image.
2. Go to http://www.motobit.com/util/base64-decoder-encoder.asp
3. Upload and convert your image.
4. Edit style and paste your converted image after "background: transparent url("data:image/png;base64," in the last element.