Getting such a library right is hard. Semantics and usage of colors in a palette are hard to generalize and there's almost always the odd extra colors that are needed to highlight something domain specific in a custom control. Also, some themes rely on gradients and mutiple subtle variations of hues and saturation to highlight boundaries, imply an ordering or prioritization, etc...

If you want to see a customization nightmare, look at the theming options in blender: the devs diligently expose every color used by the UI there. The list is massive.

If someone gets all of this right in a generic library, I'll tip my hat to them.