quite honestly, this is a pure mess. Why do you have an “appearance” tab if you cannot apply styling there? Sometimes you don’t want to go into the depths of adjusting css files. Instead apply it right where it belongs (as 1000s of other tools do it). This is not “low code” but “full code” :(
Did you check this → https://docs.mendix.com/howto/front-end/customize-styling-new#2-3-adding-custom-styling
Other things that you can check:
Hope this helps!
I would suggest you to follow this video: https://www.youtube.com/watch?v=6w4f9iV_yn4&ab_channel=MendixCommunity.