Which license is valid for gnome-gtk-themes

As I see it, every theme is essentially GPLv2. Because all of them are based on the same CSS- code that ‘someone’ once developped and distributed . So, is the commonly used license GPLv2, or should I use something else. I would like whoever to be mentioned as a contributor further down the line. But-if I understand this correctly- this would require me to find all other contributors before me. Which is sometimes impossible to do, or they are no longer available.
Next, there’s the question of the source. I make the themes myself (without any help) and only post them on this platform. And on my own computer of course. So, what source should I fill in, then?

if you wrote the themes from scratch then you choose the license. GPL v3, GPL v2, LGPL, BSD or CC etc. If you forked a theme from someone else then you should use the original license theme.

For source, you can dump the files on Github, since it’s apparently what opendesktop wants us to do. You sign in, create a new repo and upload the files.

Thanks, I think I know now. And yes it dumping them on github seems to be required.