Sounds a lot like an edge case. Most consumer applications do not need theming.
My larger point is that when you are developing a web app, it would be much easier if the framework was designed in a way that separated the concerns of presentation and behavior of the components.
Just to use Vue as it is what I am using now and had to get a bit more familiar: you have libraries like Vue Bootstrap and Vuetify (for Material Design) which have a lot of duplicate functionality yet are not compatible with one another.
If the concerns were separated, you could have components that have a common behavior and only differ in the visuals. Less work for everyone involved, easier for developers to get started, less unnecessary fragmentation and the possibility of positive feedback cycle between different design systems.