That's my question. Can the default theme be tweaked or is it something that can't be done?

Well done on what ya learnt in the other thread (Ill answer both here), your half way to being a themer
Yeah you can alter the default theme, check at the bottom of this doc.....
http://docs.google.com/View?id=dksd545_9fqhrt3dj
....I put in the old and new default themes (their actually a little of on the tint, but good enough), plus youll learn alot more about what elements there are and how they relate to each other. I took out some stuff from the original doc so you might want to look at that, the links in the links bit of my version. Im going to do another doc later on making a theme from scratch and editing already made ones (which youve already taught yourself)
If you want to make any suggestions on that doc can you make them in this thread...
[WIP] Chrome Theme Docs
....any questions on themeing, just ask

Just downloaded your doc. Monumental stuff. It will keep me busy for quite some time!