Thanks for more context!
I understand where you are coming from, but the way I feel about this currently, there is a split between web developers that embraced modern approach (eg. node.js, bundlers, js and css processors) and old ones (I just find the file, edit it, taadaa!).
css file, and then upload just that CSS file on your site via admin control panel.
Or if you want to add extra CSS bit to your site, say override the CSS for
.navbar .navbar-brand. Admin panel will have simple editor letting you add extra CSS snippet on top of already-existing CSS.
This is the course I'm taking with how customizing Misago will look like. Maybe we will have web UI for changing the SCSS variables without having to do it on your machine, don't know yet.