Background on single page

You’ll have to write CSS rules for that. In your styles.css you can try this


.page-id-27 .blog {
background: #000;
}


I suggest you try it on a local instance of your website before making site wide changes.

Take a look at this tutorial and you’ll be able to modify your site to a good extent.
http://www.wpbeginner.com/wp-tutorials/basics-of-inspect-element-with-your-wordpress-site/

Karan

Does this work?

background: #000 !important;