Function set_value 

pub fn set_value(
    path: &Path,
    key: &str,
    setting: &SettingValue,
) -> Result<(), String>

Sets one setting in the file, in place, keeping its comments and order: history_weeks, reading.size (a table’s key), account.<id>.priority (an account’s), window (every admin window at once). An empty value removes the key, so its default comes back.