I just upgraded my netbook thus: Lucid -> Maverick -> Natty. (I skipped Maverick other than as an upgrade step due to issues with Unity.)
Now, I seem to be stuck with something that looks like Emerald but apparently isn't (see note below). Compiz is running, as is
gtk-window-decorator, but my title bars aren't following Ubuntu's theme. I was using the Ambiance theme with no problems until the upgrade to Natty.
Here's a screenshot:
How can I get the default theme?
Note: I never installed Emerald, and as far as I can tell it isn't installed. There's no running process containing the string
emerald. So I'm not sure where gtk-window-decorator is getting its configuration from.
Following htorque's very reasonable suggestion to run
unity --reset made matters worse. Now, after logging out/in, most of my theme-related stuff is gone entirely and can't be changed (clicking on the themes in the dialog changes the title bar/scrollbar color but nothing else). Here's a screenshot of the current situation: