Theme placement & Management (Contrib provided ones)

We do believe it's weird that "Blank" theme is inside of "themes" and not inside of "themes/contrib". Why is this?

"Blank" is not a contrib theme, it is a custom theme generated for you when you select the Blank template. You will see there is no project for it, because it's not a contrib project.

When you rename "blank" to your own theme, then you get warnings that it can't find the blank theme.

Renaming a theme that is already installed will cause issues, yes. We should add some documentation for this, right now it just links to the starterkit theme docs which don't really apply because you don't get to provide the name.

Besides that it's weird that when you want to alter the "Mercury" theme that you need to copy the CSS files to the root of your webproject?
Why is this?

Isn't it cleaner to sub theme or to have an interface where you can alter some CSS properties, rather than dumping these files in the ROOT of your project?

Why yes, it would be much cleaner :) Modifying CSS tokens within the Canvas editor is in their roadmap, but it doesn't exist yet and in the meantime that was the best we could do to allow for CSS overrides without having to maintain a custom theme.

Created #3587497: Create some documentation for the Blank theme for the blank docs.