We did this, and this is quite simple for one site, but our approach proved difficult to repeat further.
In any case, you may have to work with the file:
We used to use plugins, but for more rational use you need to do on components. They do not require a restructuring of Discourse. And you can change everything on the fly.
The links above are a 100% solution to what you need. Maybe not ready, and you have to work with CSS (html).