How to reset GNOME default theme?
I wonder and I needed it sometimes. Some time we install many themes and it gets worst.
DO NOT
rm -rf ~/.*
You will delete your configurations for every application in your profile, not just the gnome theme settings.
Instead, do:
rm -rf .gnome .gnome2 .gconf .gconfd .metacity
If by default theme, you mean the one that came preinstalled with 10.04 and 10.10, that's called Ambiance, and it can be found in System -> Preferences -> Appearance. The installed themes will be listed here in alphabetical order.