What server file to edit to change default theme? (upgrade to 2.3 rc1 with missing theme (x_treme) - can't see anything now!)

Howdy.
I had 2.2.5 with Pedro Gilberto's x_treme as the default theme (latest version, 1.06 I think).
Now I've upgraded to 2.3 rc1 and when I go to the gallery the page is blank and this is the URL that shows in the browser:
<mydomain>.com/gallery/main.php?g2_view=core.ShowItemError&g2_problem=missingTheme&g2_itemId=7
(the install did show some x_treme them files where missing, like about 1400!)

Is there a file I can edit on my server to change the theme and fix this problem?

If not, I have to roll back to a backup set of files and repeat the whole upgrade process again after first changing my default theme, something I'd like to avoid.

Regards,
Colin