Can we allow CSS for users? So that everyone, or at least higher TLs, has ability to chance how things look.
Is that even technically possible (without using purpose made theme/CSS-component by admin)?
What would be risks — other than a user breaks this for him/her/itself?
That would take off some pressure what users like to see, or mostly not to see.
Yeah, it will at some point, especially since Discourse’s HTML code changes from time to time.
I used at least one website that proposed this feature. I wouldn’t advise implementing such a feature unless there is a very specific need (a CSS enthusiasts community for example?).
A missing bracket is enough to make the whole site display: none; 
Just to clarify, the CSS would only change for themselves locally and not globally? If that’s the case, couldn’t a browser extension be a solution?
The only problem with those extensions is they often dont support sass and full themes. I’ve used a SCSS to CSS compiler before to get a custom theme on another forum, and it’s not amazing.
My idea: A theme called “custom theme” on the theme that would bring up a menu that would allow you to use a custom theme from the theme category on this site or your own themes from https://discourse.theme-creator.io/
If locally means just that one user then yes.
But then there would needed something like if user = X then do CSS and I don’t know if that is even possible.
Yes it could. How would I install such extensions in iPad or any android?
Have you looked at Tamper Monkey? Tamper Monkey scripts allow users to change how websites display info.
A member of our community make one when discourse didn’t have the mute user option. He added a method to mute/hide specific users; decorate Op & Staff members and added notes to certain users by group etc…
If you can install Firefox iirc Tamper Monkey is an available extension.