I’m asking because I’ve installed so many themes that I lost overview of them. I wanna clean up and safe some space but I’m not sure if I can just delete this folder and recreate it with my favourite themes. Is this advised or will this break my themes + my installled one?
generally user installed theme would go into ~/.themes directory so deleting it is safe but make sure your current theme is not in that directory or you can switch to a default theme, delete the directory and install it again.
similar issue 
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Mainly, i would first rename something to xxxxold, when you want ‘remove’ it sooner or later.
After you moved it out of the way, and afterwards all still works, then you can delete it.
Are the default themes also in that folder or are they in another folder?
default themes are in /usr/share/themes
Default themes, along with any themes installed by regular packages, will not reside there.