This PR converts the `ThemeRegistry` type into a trait instead of a
concrete implementation.
This allows for the extension store to depend on an abstraction rather
than the concrete theme registry implementation.
We currently have two `ThemeRegistry` implementations:
- `RealThemeRegistry` — this was previously the `ThemeRegistry` and
contains the real implementation of the registry.
- `VoidThemeRegistry` — a null object that doesn't have any behavior.
Release Notes:
- N/A