https://rarlindseysmash.com/2022/11/21/mastodon-quick-start.html okay wrote this up, there's some things I couldn't figure out how to screenshot but hopefully it helps folks.
I was just typing into the custom CSS field and loading it - I didn't realize it was CSS only and not SCSS because I tried to initially just overwrite the variable values, but that failed. I could probably go that direction, but I wanted something easily shareable with other folks.
https://gist.github.com/LindseyB/8af0e11c837e852d26a9239f0718b4d4 is what it is so far. It can be greatly cleaned up still but it's a start. There's some definite contrast issues still, but since it's just me *shrug*