If you can devote all of your weekends in a month into engineering CSS3 and the structural nature of XF2 default templates, then you might not need a framework to do it.
Anyway, this might be too geek to some people, hence commercial framework add-ons to save their time. (I don't like the taste, hence modding my style templates by my hands.)
Less extends CSS with dynamic behavior such as variables, mixins, operations and functions. Less runs on both the server-side (with Node.js and Rhino) or client-side (modern browsers only).