I would like to use SASS to build a nice theme so let's try jekyll-asset-pipeline.
Update: Using other gems is not safe and builds aren't done by github.com, you must build & push to gh-pages manually from this point. Duh!
Found a nice Rakefile from @ixti to generate & push to master.
As github pages for users must be sent on master when generated manually,
I have created a new branch source to keep track of my changes.