adding classes and ids to posts

If I wanted certain posts to have a background color of yellow (for example) how could I add clas=”yellowbkg” to these posts?

Ultimately, I’d like to customize my site homepage into less of a blog-style chronology, and more closely control the display of excerpts on the page, having certain posts in one column, certain posts in another, and certain posts spanning both columns. See this diagram as an example (disregard the sidebar comments): https://www.cultivatecreativity.com/images/SitePlan.jpg

Once I know how to add different classes or ids to my individual posts, is it just a matter of getting the css written for the positioning?