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.