People have already modded Scratch to have more features, some of which are already in Snap!, and though some of the general blocks like the unicode functions would be relatively easy, it would also require modding in first class lists, functions, and blocks.
Not that I know of, though the closest you can get to the Scratch 3 style is turning on Flat Design (editor > settings > Flat Design) and having block size set to 1.6 (editor > settings > Zoom blocks).
Yeah, I was going to ask @samq64 just how close to Scratch 3 he needs to be. If you just want it to look ugly, Flat Design might already meet that need. If it has to be the exact shape of new Scratch blocks, that's harder.
Changing Scratch to become Snap! would be a major undertaking. The most obvious Snap! features, such as individual blocks or zebra coloring, aren't the most important ones. What really makes Snap! Snap! are anonymous functions. That's actually easier to design in in the first place than to retrofit into an existing system in which functions aren't first class.
But honestly, I'd like to talk you out of the whole idea. When you're learning a new language, don't start by trying to make it just like something you already know. Let it be itself! Imho.
Yes. But more than that, I think almost all web sites are ugly, because they work only on white backgrounds. If Flat Design had a dark background, I wouldn't hate it so much.
One reason we have Flat Design as an option is to make it possible to embed a Snap! project in a larger web page, without making the dark background stand out so much.
I've been complaining about the non-support of dark backgrounds in web pages pretty much as long as there's been the web, but finally the authors of browsers are starting to catch on that a bunch of people set their backgrounds to black and then have to deal with black-on-black text on nonconforming sites that set the foreground color without also setting the background color. So I'm hoping I can stop griping about Flat Design soon.