Folder color in thumbnail mode

[lead]

@leo

(unless we put in a separate color option just for the thumbnail folders, I guess).

Found this here: Big Yellow Icon in Thumbnail mode { Picture Included }

@jon

It's actually drawn algorithmically (because it has to scale)

So, it doesn't look like a very hard request

I could make it white, so opus labels don't mix the colors, maybe.
I would be nice to be able to make it a solid color.