How do I change the blog post background to a HEX color in CSS?

[MLB1975]

I have a custom theme and therefore Weebly couldn't assist me with coding help. The background to my blog posts is white. I have been all over the web searching for how to enter the CSS coding to change it to a specific HEX color, but I can't get full instructions on where to enter it. I'm hoping someone out there can help. Thank you.

[Adam7]

Can you post a link to your site, @MLB1975? I think it'll be easier for people to figure it out if they can see the live site. Thanks!

[MLB1975]

I had posted it a few days ago and have no idea where it went. My apologies.

My site is www.headmiddlechoirs.org

I'm just to find a way to make the background for blog posts a color instead of just white.

Thank you.