The thumbnails will not be dynamic. Unless I misunderstand what you mean by dynamic.
They are unchanging images just like any other asset thumbnail.
So they need to be easy to create and reproduce by anyone. Simple viewport screenshots of a brush stroke are the goal.
I agree that there is no perfect background value to support all colors.
We could use a transparent background. But then all themes are forced to use a background value in the asset shelf/browser that works. Because the thumbnails will not be automatically inverted to keep them readable.