This PR changes the theme loading to use the JSON themes bundled with
the binary rather then the Rust theme definitions.
### Performance
I profiled this using `cargo run --release` to see what the speed
differences would be now that we're deserializing JSON:
**Before:** `ThemeRegistry::load_user_themes` took 16.656666ms
**After:** `ThemeRegistry::load_user_themes` took 18.784875ms
It's slightly slower, but not by much. There is probably some work we
could do here to bring down the theme loading time in general.
Release Notes:
- N/A
5f1dcb76fe