I've always liked the Oblivion theme for GEdit (and other GtkSourceView based applications). It's based on the Tango palette and I find it to be much easier on the eyes than most high contrast dark themes. To use it as many places as possible, I created a similar theme for Xcode and TextMate. Appropriately, I entitled it "Morrowind". It looks like this (in Xcode 4):
Download the theme. Place it in:
~/Library/Application Support/Xcode/Color Themes.
Apple changed the theme format in Xcode 4, so I had to do a small amount of porting. Download the Xcode 4 version and place it in:
In both themes, I have set the font to Panic Sans. This should automatically fall back if you don't have the font installed, but if you'd like it, you can get it with Coda.
Download the TextMate theme. Double click it.