Yes, of course.
These themes you linked to have never been compatible with Mypal 68.x but with the old Mypal 29.x. You have to look for themes compatible with Firefox 68.x (Firefox Quantum), for example, here: https://addons.mozilla.org/de/firefox/themes/
But do not expect to find complete themes as it was the case in the legacy browsers! They are almost gone. Only light ones are still available. Regarding your issue, you seem to mix up legacy browsers like Pale Moon, New Moon and Mypal 29 with Quantum based browsers like Firefox 68 or Mypal 68. You can read more about that, for example, here: https://msfn.org/board/topic/183657-mypal-68-in-windows-xp-custom-buttons-and-extensions/ There, I have listed different ways to customise Mypal 68. And of course, you will also find information in many other threads here on MSFN and everywhere else in the WWW. Search for Firefox Quantum themes!