This property isn't needed because it's managed by libcosmic

A subscription for theme changes is not necessary since you can get that from libcosmic directly.

You can use cosmic::theme::is_dark() instead.

This variant can be removed because libcosmic is already subscribed to theme changes on startup

Shouldn't need to write this file, or assume that dark is the system default.

It would be better to toggle it through libcosmic

use cosmic::cosmic_config::ConfigSet;
use cosmic::cosmic_theme::ThemeMode;

ToggleTheme => {
    if let Ok(config) = ThemeMode::config() {
        config.set::<bool>("is_dark", !cosmic::theme::is_dark());
    }
}

Better to cache the icon handle instead of the name

widget::icon::from_name("icon-name").handle()

Shouldn't write to files directly here. See cosmic_config below.

This file can be removed. In the future you'd want to use the application's core to subscribe to a cosmic_config interface.

self.core.watch_config::<MyAppConfig>("com.github.MyAppName")