Hi David ! Thanks a lot <3
Those details in the background are really simple to achieve, you can do it in only 5 minutes !
I don't know if you are used to Unity3D, but there is a component called Particle System, I only add it and tweak some values to get this effect.
No code needed for this one !
Then I duplicate this component so that the last boss has more particles than the first one for example.
I can be more specific if you want ;) 