I’ll check the White Theme when have some time.
I can load the White Theme without problems and boy, is it White. Whiter than Default.
Are u sure you’re using Window > Themes > Load Theme from file, and then Restarted Scrivener to actually see the Theme?
The only problem I can think of is it you named the Theme or the folder containing it “Default” because that might clash with the… Default Theme.
Send me the Theme and I’ll check it to see if I can find anything wrong with it…