You *can* use .less files in extensions, but I see 2 issues:

1. The less gets compiled every time you start Brackets. This slows down startup time, but even worse provides an example that other people will copy and make it even worse.

2. You can pre-compile the .less into .css, but it's a step you have to remember to do every time an edit is made.

I just thought I'd mention putting colors in a comment. If you don't think it's necessary then I'm ok with it.