I wonder and I needed it sometimes. Some time we install many themes and it gets worst.
4 Answers
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.
Assuming you mean the theme that comes with a default install of Ubuntu, go to System->Preferences->Appearance and click on Ambiance (dark theme) or Radiance (light theme).
If you instead want the non-Ubuntu-default theme that GNOME ships by default, click on Clearlooks.
Use the following commands to change the theme. Ambiance is the default theme as people have mentioned above
gsettings set org.gnome.desktop.wm.preferences theme "Ambiance"
gsettings set org.gnome.desktop.interface gtk-theme "Ambiance"