First, are others also affected by the change? or is it just you? Did ya try a different browser?
Might clear your browser cookies and cache and purge the cache on the server via URL: http://yoursite/admin/purgecaches.php
Can override theme by adding:
$CFG->theme=standard;
or one of the 'standard' themes that came with the original package:
afterburner
anomaly
arialist
binarius
boxxie
brick
canvas
clean
formal_white
formfactor
fusion
leatherbound
magazine
nimble
nonzero
overlay
serenity
sky_high
splash
standard
standardold
You could also insert debug lines in the config.php file to see if there is any error reported with that 'white screen of death'. With as much trouble as you are having, suggest leaving them in the config.php but comment them out (like below) when the latest issue is resolved. Might also just comment out the theme line as well.
// @error_reporting(E_ALL | E_STRICT);
// @ini_set('display_errors', '1');
// $CFG->debug = (E_ALL | E_STRICT);
// $CFG->debugdisplay = 1;
'spirit of sharing', Ken