The FetishFramework is designed to do this.
95% of the entire UI can be reskinned by changing a handfull of images and 1 Styles file. Any functional mods skined for Fetish will be updated with the new skin. Theres already a couple alternate themes available with more on the way
Since most grphics and style have been consolodated into their own namespace, default style and images from SOE aren't affected, which minimizes breakage when SOE make changes.