I have found out that if you are using your computer for any reason (ie:watching Video, Moving the Mouse) it will not change the System Theme to the Dark Setting. If you just let your Computer sit and not use it for a couple of minutes or sign out or restart the system it will change on it own. This has been this way since I upgraded to Catalina and all the updates that has come out from Apple since then. I just chalk it up as a work in progress and Apple does not think it a priority to fix. Hopefully when Big Sur comes out it is fixed. My IOS Devices work just fine with it!