custom css using shortcode only

Hi,

I’m attempting to use this plugin only with the shortcode, I’m not using the widget at all. I’ve added styles_default=”false” to the shortcode, but where do I add my custom style?

Also, is it possible to use a custom css file inside my theme directory or something?

Thanks for any help.

https://ww.wp.xz.cn/plugins/recent-posts-widget-extended/