The devs know they need to make a brand new paint editor, they just have more important stuff do work on.
The current colors look good, but if you're used to a lighter color, it may take time to get used to. Plus, if you want a light design, try enabled the "flat design" (settings > Flat design), which is basically just light mode.
Tutorials are not going to happen, because they don't have the resources, however there is a reference manual, and there's a course called BJC (Beauty and Joy of Computing), which teaches programming in snap.
Extension blocks already exist in snap as libraries (file > Libraries).
The reason there's not more costumes and sounds is actually due to how the current costumes and sounds dialogs are very slow when they have a lot of costumes and sounds, although that can be fixed with some optimizations.
This is also something you should probably read.
Honestly I agree with this, because it's in the open project dialog, which can take some time to go into, and select the currently opened project. I would actually still not put a button in the header, I would instead put it in the cloud menu, because that would give another use to the cloud menu than just loggin in (and changing your password).
I kind of agree with the snap guidelines, however I think that should only be shown the first time you share a project.
I don't think there should be tips to getting featured, because snap is not a popularity contest (which is why there will never be likes, and you can't see how many followers you have), and you should not strive to get on the front page, you should instead just make projects you're passionate about.