What I've also found is that the coloring of the skill tree is too bland. It's very hard to see the difference between a fully bought upgrade, an unavailable upgrade, and an available upgrade. The last one can be spotted a bit easier because of the animated + sign, but I can't make out the diffence from the colors. I'm not color blind, but my eyes aren't great either. To me the difference in colors is just not good enough. In earlier versions the skill tree looked much better. Not sure why you changed it.