The start of custom CSS is here.
To make custom css just make a text post anywhere. I suggest posting it in /s/customcss
To make your css more legible and so it doesn't get interpreted as markdown I suggest opening a code block in the post
YOUR CSS HERE
Then you can simply go to settings using the gear icon in the top right and paste the link to your post, or to someone else's.
Escaping your css to make it more legible is 100% optional.
It is not universal yet but will be. It currently does not act immediately but will.
Also there will be the ability to #include another post's css inside of your own.
An example CSS that you can use right now is here: https://matrix.gvid.tv/c/test/5hua5hzsiF