Make bg color partially transparent?

Hi all,

Here is my link:

https://vintageartreprints.com/wp/

if you scroll down to the main content area (Latest Products), you’ll see I’ve made the bg color brown. However, I’d like that brown to be partially transparent so that the wrinkled paper bg shows through. Can anyone tell me how to do this? I tried a few things in the CSS but they’re not working. Any help is appreciated!

Gary