Not sure how widely known this is, but came across Lavish Bootstrap, which allows you to generate your own Bootstrap color scheme from an image. I did notice that it fixes the width of the categories, so you will need to go through and delete all the .col classes from the custom. But could be useful for those that can't be bothered to create their own themes and just want a little colour change. (You could probably put a screenshot of your existing forum in and get a theme out that looks similar with a few option tweaks.
You can either create your own theme using the less files, or just drop the CSS into ACP>Themes>Customise, both work.
Anyway, have a play here, and then maybe someday when users can pick their own themes, someone could shoehorn this sort of functionality in with it via a plugin.