You can only switch themes with an account as far as I'm aware, but all 3 kinda suck in their own ways

I agree that the current default isn't very nice to use on desktop nor appropriate for Heyuri - MonoBook seems like the best choice for the "classic" wiki appearance, but it overrides certain CSS properties if you try to style preformatted text for example, so you can't get AA looking quite right (a relatively minor issue that nobody other than me will ever run into, but still!)

Vector doesn't suffer from that problem as much, but it has a similar interface issue to Timeless (the current default theme) where the sidebar is hidden until you press a ghey hamburger button, despite the fact Vector doesn't even properly support mobile viewports (yet MonoBook does!?). Plus it perma-hides the top left logo, which is teh lamez0rs

So yeah, I'd probably vote for MonoBook to be the default even though it's not perfect