public function setDefault($theme) : Theme
    {
        if ($theme instanceof Theme) {
            $default = $theme;
        } elseif ($this->has($theme)) {
            $default = $this->get($theme);
        }

        if (!isset($default)) {
            throw new NullValueException("Cannot set default theme when theme does not exist");
        }

        if (!$default->verified()) {
            throw new \InvalidArgumentException("Cannot set default theme to an unverified theme");
        }

        return $this->default = $default;
    }