public function register(string $id, callable $callback): self
    {
        Internal\Themes::$registrations[$id] = $callback;

        if (Config::$initialized) { // Initialize to add asset dirs
            $currentThemeID = Internal\CurrentTheme::getID();
            if ($currentThemeID === $id) {
                Internal\Themes::initialize($currentThemeID);
            }
        }

        return $this;
    }