Code for text area Snapshot theme

Could someone please help me with setting up a grey text area background so that my posts are readable when I have a background image. Here is my website and it’s the Snapshot theme. If you could advise me on the CSS code I need to use and where I need to put it in the CSS file, that would be great! Thank you!

http://sjulab.com/backoffice/uncategorized/first-official-test-post-for-lab-200/