That browser, just like all Chromium based browsers, get that setting for free from Chromium making that a thing in the first place. Some browsers choose to bring up that option in settings, but it's still just the same thing as the #enable-force-dark flag. (What's really unfortunate is that Firefox doesn't have anything to match that feature, and all of those "auto dark theme" addons are just awful, they're less reliable and look worse.)