First page already looks way more complex and flimsy then Twitter.
Then I click on 'or create an account' and I am redirected to mastodon.social. WTF?
Pinafore: I don't understand what I am supposed to do with it.
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.
It looks like a bootstrap theme but with all spacings done wrong.
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.
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.