I don't know what your mobile OS is, but Dark Reader works just fine for me on Firefox Nightly for Android. Mozilla is still in the process of adding all of the web extension APIs to mobile Firefox, so you need to go through a few steps to set it up, detailed here:
https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...
Dark Reader just shows up in the addons section for me in fdroid Fennec, and Firefox mobile. I don't think you need to do anything special to get it to work.
Oh, thanks for letting me know. I have been using that little trick since they had first released that feature for unsupported addons and there were very few available then. That is good to know it is available in the supported list.
