Eek… the easiest and best long-term way is to cut-paste the style files into your own style and just change the textures, fonts, whatever, there. Most of them inherit the background image from the same one or two definitions.
Then just load that style and set it as the default instead of the build in glass style.
Edit: glass style for core:
Add on styling for the lemur proto stuff:
For your own, you can merge them into one file if you use lemur proto.