OpenUI for a standard way to have themes and GUI/TUI features across platforms

In a discussion about a program that found it hard to have cross-platform themes, the idea was raised that Khronos could make an OpenUI API. It would make it possible to easily have theming across different OSes or at least some desktop/mobile environments that support it at first. I understand it has difficulties, notably that some OSes might feel they want to retain full control, possibly not necessary for benign reasons.