CSS to add background behind posts

Hi. I’m quite aware this query has probably been posted so many times before. However, I’m aware that you can somehow get either a solid background, or an image behind the posts on my wordpress. My website can be found here (https://footballgfx.net/). As you can see, without anything behind the text it looks pretty dim, so I’d like a light grey behind it.

If you’re not quite sure what I’m asking, then say and I’ll pipe something up in Photoshop. Thank you.