Board Thread:Support Requests - Designing Your Wiki/@comment-1789085-20140112162302/@comment-168424-20140113213253

Theme designer should work fine. However, it does not let you upload any image to use as a background and has some restrictions.

How big (in Kb) is the background you're trying to use?

It is usually much easier to get Theme designer to work correctly first and then work on the CSS alternatives later.