bleach wiki? as in {{#NewWindowLink: w:c:Bleach|this Bleach Wiki}}?
Have you checked whether your bg image is below the file size limit(100kb)?
Hi, I beleve it's because Bleach Wiki's Wikia.css included a background definition that used a file that no longer exists (File:BleachWikiBackground10.png), so it was probably never taking from ThemeDesigner.
I have commented out that bit of CSS in this edit: [1]
Once the cache clears, the ThemeDesigner-selected background should appear.
if you want to us css and the theme builder, use this link for the image:
http://bleach.wikia.com/wiki/Special:FilePath?file=Wiki-background
(theoretically)
Muchos Gracias!!! Always so helpful staff!! :D