Ask HN: How to Contribute UI Design to OSS

I'm the lead designer of an unlimited UI design service for web and mobile development teams (http://fairpixels.pro). I'm looking to give back by contributing more to OSS with UI design work. I'm particularly interested in tools that have a decent userbase, but for some reason never had an opportunity to get help designing a better User Interface.

Question: How do I find OSS projects of this nature that need UI help? Suggestions/introductions are welcome.