Closes https://github.com/zed-industries/zed/issues/44091

The issue here was that we were missing a proper default for
`ThemeSelection`. This is a disadvantage of not having the defaults in
code but actually in the `default.json`, so this here basically copies
the default from the `default.json` into code for the settings UI to
work properly.

Release Notes:

- Fixed an issue where the theme was not configurable from the settings
UI if no theme was configured prior.