Changing web pages to all black

I would like to know how to change pages on my website to all black. I would like to know how to manipulate the appropriate code to do so. Any help would be appreciated. I’ve submitted a ticket on this already; however, I think the way I may have worded led the team members to believe that I was going to figure out the answer. I haven’t figured out the answer. During my html dog website tutorial training the notepad stopped translating the css into my web browser as soon as I started learning from the css pages.

I’m simply trying to get a faster answer than going through all the course work of css. I know it is eventually inevitable. Any help on this will be terrific!

P.S. I’ve tried getting support from my web hosting provider. Though they are extremely helpful, they aren’t readily knowledgeable in this area.

https://wordpress.org/plugins/simple-custom-css/