Some MORE data
Sorry, it keeps trickling in.
After my last attempt at a theme change and internal server error which I captured for my last post, I logged back into the themes page and "Voila", my theme was changed back to the original. When you click the new default theme, the highlight doesn't happen, either.
Attempting to change the header image of the current theme also produces no change in the preview (though I can tell it's trying by blinking), and a crash on saving.
The error is displayed on page unload.php
(ofthefield.com/resources is the installed directory.)
Finally, the header change doesn't stick, but the upload of the image works.