I’ve attempted to make custom theme 2 years ago. Then i found some issues with hardcoded colors etc. So clearly UE4 required a custom theme support embedded in the engine. But I just had no enough knowledge back then. Probably still have no.
https://github.com/S-ed/UnrealEngine/tree/MinimalUI
To be clear. I think UE4 UI is not bad. It’s just everyone has own preferences.
But would be nice to have some kind of loader to automate this stuff.