I am willing to contribute my free time to open source projects but most projects on Github require software engineers.
Who could point me to a couple of open source projects on Github where I can contribute my visual design and CSS skills?
I'd like to introduce you to the Indieweb.
This is a community of people creating open source tools that enable people to maintain their own identity on the web without being dependend on social media. It's as simple as having a private website but without loosing the social aspect of the networks.
It is build upon a lot of principles, the most important one is "own your data". If you post something exclusively to a social network, the platform owns it and can decide how to present it, and who can see it. The own your data principle means, that you post on your own site and syndicate your content to every platform you like. And even facebook for example decide to take it down, it won't be lost.
There is a lot more, you can read about it if you're interested following the link above.
The thing is, most of the contributors are super programmers but with a lack of all the visual stuff. Some of them often ask for help.
The GitHub index is: https://github.com/indieweb
You can contribute to the individual projects or you can join the IndieWeb and co-influence the whole agenda. Everything is build upon a really great code of conduct and all the people are super friendly and open minded.
If you're interested, don't miss to join the slack or irc channel.