There is Fork. But sadly, it is not available for Linux. Git-fork.com
Fork is only “free” in that the evaluation period is indefinite. This is generous and clicking through the nag isn’t a huge deal, but I develop on both Linux and Windows and I need a client that supports both.
Ah, sorry. I didn’t see that you require it to be free. It is also not open source IIRC.
Open source is a definite plus, but tbh not a requirement for me. Actively maintained, free, Windows and Linux, and simple. Oh, and it has to have a dark theme 😄