As described in another issue, upgrading from 12.04 to 12.10 may break the Ambiance theme, causing many of the menus and controls to display in a strange grey-on-grey state.
I have checked another user account and the theme appears fine there, so it seems to be a user-specific issue.
What can I do to reset my theme settings to the standard for 12.10? I assume there is some set of config files in my home directory somewhere that are customizing it incorrectly.
(As the linked issue was closed for being "too localized", I'm hoping a general fix can be provided for many similar theme issues.)