I’ve copied some files for custom settings from my desktop to the freshly installed notebook.
Both systems with Linux 612 and Gnome 48.
While Gnome Tweaks on the desktop shows these in the corresponding drop-down list (like it had done for the last 5-7 years) it doesn’t on the notebook.
It should find them e.g. in ~/.themes or ~/.local/shared/themes but it doesn’t.
Any idea why? I don’t have a clue. 
Hi @nitja,
Not using Gnome myself, I’m a Plasma user. I’d bet good money on ownership and/or permissions.
There aren’t any differences in ownership of permissions between the 2 machines.
And I meanwhile found that it’s rather about the system accepting the corresponding settings than finding them.
Anyway, that notebook will not be ready to accompany me on my next travel (starting tomorrow). 
Will continue the (re)search for solving this issue (an those to come while trying to get the system to a productive state) after my return (next Wednesday).