It’s not difficult anymore, if I recall correctly (I did it for some static site generator template a few months ago).
For the forum it was a little tricky because I had to do some tweaking to get our dark and light themes working side-by-side together with the small tweaks I made in the #gaming category in the beginning.
Once I figured out what was causing the CSS bug, though, it was easy enough to fix.
I see it too. Seems on mobile the Discourse image, which lives in the left margin on the desktop, doesn’t play nicely. It’s fixed to the top of the page, so should only be obscuring the first post or two, but yeah, it’s big ugly. @SIGSTART may know what causing it - I’m guessing another small issue with the recent update.
Auto colour/light mode switching nuked the workaround I had in place to fix this issue. For now I’ll just remove Bob* (I can’t remember what @DieGrootHammer named him all those years ago) until I can figure out how to put custom CSS in the theme that loads based on your system theme.