if you mean you want to change the content background on a single page, you should know how to do that, and we have a tutorial in the old weaver tip section 🙂
Just use a rule with the page ID#
.page-id-2690 #content {background-color:rgba(255,255,255, 0.8);}
If you have Plus, you can simply put the rule in the per page style box of the page editing page
#content {background-color:rgba(255,255,255, 0.8);}
If not what you are trying to do, then you need to elaborate