Basically I try to make my own borders so that colors can be personalized without much work.
I imagine the background color as the first layer and above it the partly tranparent image. This get's really cool if one can not only use bitmaps, but png with alpha channel support.

The themer makes only one set of graphics and the background can be customized in a different file by the user.

But maybe this all does not work and I am dreamer ;-)