public function set($theme, $group = null)
    {
        if (!is_null($group)) {
            $this->currentGroup = $group;
            $this->currentTheme[$group] = $theme;
        }

        if (is_null($this->currentGroup)) {
            throw new \Exception('Current group cannot be null. Use Theme::setCurrentGroup($group) or Theme::set($theme, $group)');
        }

        $this->currentTheme[$this->currentGroup] = $theme;

        return $this;
    }