There have been a couple of changes (I think both in 2020 but I only started using Joplin seriously in late 2020). One was a UI/design refresh and the other was changing the older Ace editor to the current Codemirror editor so I guess this will have broken quite a lot of the older themes.
You might want to have a look at the newer posts in Share your CSS and see if there is anything there.
There was also a change (and a lengthy discussion) about the changes that were made to the syntax highlighting on the markdown editor. The changes were made for themeing consistancy but there are others (like me) who subscribe more to the "markdown is code" metaphor where I prefer to have the syntax highlighting etc. albeit with the "hybrid" approach of dynamic text sizing for headers etc.