there are a few alternate web user interfaces that work with the Mastodon client API (which Pleroma also implements); halcyon and pinafore being two of them. To be more specific, Halcyon's UI is Twitter-clone.
Brutaldon is even more pared down skin (and offers an even more Web 1.0 brutalism beyond that): https://brutaldon.online/
Most of the skins redirect to mastodon.social on the Create Account link because they don't host the accounts themselves, they just offer views for whatever account(s) you do have.
Sorry, but that is not brutalism. That is just a badly designed page. Small grey font is pretty much the opposite of brutalism. So are small pastel colored buttons with rounded corners.
It looks like a bootstrap theme but with all spacings done wrong.
Brutalism is in the eye of the beholder, of course, and any talk of Brutalism on the web is an exercise in analogy given that the core mantra of architectural Brutalism was to stay "true to the materials" and for pixels it's hard to be "false" with them.
The variation of the theme that just uses HTML2/3-esque TABLEs and table borders is definitely more directly evocative of that spirit, stripping away CSS for just the brutal truth of classic HTML design. I tried to find a direct link to that version of Brutaldon's theme, but am not sure where to find one, and searching one's Mastodon feeds from weeks seems to be tougher than I expected.
Halcyon and Pinafore are both just front-ends for existing mastodon accounts (like brutaldon).
Pleroma is a different technology, but federates the same. As is Misskey; depending on what you're looking for in features either of those might do it more for you (or the glitchsoc fork of mastodon). (and theming is a thing, of course, regardless)
It's been posted here before, but http://fediverse.party/ is a nice little overview of the federated social web (over ActivityPub, I believe); https://the-federation.info/ is broader. Both have some small issues but are nice starting places.
My exact reaction. I have tried using Mastodon, but that thing is designed by engineers with no common design sense. Just sticking to browser defaults would be a huge improvement. It looks like something I should use to impress 15 year olds with.
