On toolbar click on a wrench key, on the most left there is an icon with 3 stripes. It will open Tree Tabs options.

Download some themes from here:
https://drive.google.com/open?id=0B3jXQpRtOfvSelFrTEVHZEx3Nms

When you change theme effect is not immediate. Just reload sidebar, for example change sidebar to bookmarks and then go back to Tree Tabs.