Hello, Rachelle!
This option is located in Customizing->General->Body menu. https://prnt.sc/sk3g9i
Also, you can use the Custom CSS code to set your Page Content area semi-transparent.
(insert the code into Customizing->Additional CSS)
div.content-wrapper {
background-color: rgba(255,255,255, 0.5)!important;
}
PS: Change the opacity level as you need (change the last number from 0-1)
Regards