public function setOverride($theme = null) : ?Theme
    {
        if ($theme === null) {
            return $this->override = null;
        }

        if ($theme instanceof Theme) {
            $this->override = $theme;
        } elseif ($this->has($theme)) {
            $this->override = $this->get($theme);
        }

        if ($this->override === null || ($this->override !== null && !$this->override->verified())) {
            $this->override = $this->default;
        }

        return $this->override;
    }