I answered this one in an earlier thread:
http://forums.omnigroup.com/showthread.php?t=1708
Basically you need to change the OS system theme for the 'image well' (the sunken box you sometimes get around images) in order to change this. A while back i ran a shapeshifter theme just to get rid of the gradient.
The status bar can be modified a bit. I wanted to have a gradient on that (like xCode) but any custom images that get applied to the .nib file will sit on top of the loading and status bar messages/icons. There's no way to get a custom image to go below that, unless you're the Omnigroup of course! I checked with them about a month back and it was confirmed, so I've given up on that one. It could be that someone clever at cocoa could write a SIMBL or InputManager plugin to do this, but it doesn't seem worth the time. Hopefully OG are working on am updated interface for OW 6 that brings it up to speed UI wise with the rest of their gorgeous looking apps.