I did some more work on the background shader. Especially the use of radians instead of the usual degrees. The Firefly math nodes do not offer a solution here. But the Math Node from Cycles does. It has a much wider range of functions. And here we also find two functions to convert the rotation direction from radians to degrees and vice versa.
I have also uploaded the whole thing as a free item. This should be unlocked soon.
The whole thing now looks like this: