Function load_themes_dir 

pub fn load_themes_dir(
    dir: impl AsRef<Path>,
    cx: &mut App,
) -> Result<Vec<SharedString>, ThemeLoadError>

Registers every *.json file in dir (not recursive) as a theme, without activating any. Files load in name order, so the result is deterministic; the first invalid file aborts the load with its path, and the files before it stay registered. Returns the registered ids.