I download and applied succesfully ArduinoDark theme from a .zip downloaded somewhere online. I tried to zip other themes from https://github.com/AbhishekGhosh/Arduino-Themes yet unsuccessful. After a lot of trial and error, found out that uncompressing the functioning .zip file and compressing it again renders it invisible for the IDE. Even under the same name. Changing the name while to the .zip downloaded file has no effect, IDE recognizes it after reopening preferences.

Another thing I have noticed is that the un-compressed and re-compressed file is quite larger being 70kb the downloaded one and 86kb the re compressed one. Maybe something about apple zipping protocol?