The short answer is you don’t; I think I was probably the person who told you that a while back.
8.0+ is using a custom-skinned version of the Synthetica Look-And-Feel. For performance and other reasons, this LaF effectively renders and paints entire images on components, not single colors that can be easily controlled by UIManager properties.
So the way to ‘theme’ a Vision application would be to obtain a Synthetica theme .jar and apply it to the Vision client - except you can’t do that, because 1. you don’t have a Synthetica license and 2. you don’t have the ability to run custom code on startup (outside of, I guess, a module, but even then it’s probably going to initialize too late).
I would really strongly recommend abandoning this approach. Tell whoever needs to be told that Ignition made it impossible to customize in 8; that’s close enough to the truth.