Loading dark theme css-files results in switching to the standard light theme

[SN-CH]

I am not sure I understand how this theme css-files are supposed to work.

I downloaded one of the files (specifically, DARK THEME-jabrefdark-greytext.css) and entered its path into File / Preferences/ General / <the untitled field for file path> (selected Custom, of course). It had an unexpected result of changing the theme to... standard light one (that makes me think it is a fallback solution in case the css ist broken).

I tested it on Jabref 5.12 on Windows 10 and Jabref 5.7 (different path to the css field, but the same idea) on Lubuntu 20.04 only to get identical results.

Also, I checked some other dark theme chosen at random (not sure which one it was). Again, the same result.