Will probably not be done, unless maybe glitch-soc/tootsuite adds them. And considering that they have themes (css ones, baked with the frontend) I think they don't, we just need an interface for them.
By clicking on the triple-cog, present on top of the composing column and around to middle of the getting-started column (last one, displayed on start).
I'd love a way to manage filters in particular. Currently it seems there's no way to add them at all, even though there's settings to change how they're displayed!