Hi there @frein --Looks like you're trying to set the background color for the page on just one of the Container stacks. What you'll want to do is to set the Site Background color in the main Foundry stack. Click on your main Foundry Control Center stack and then find the Site Background settings, as shown here:
Choose Color Picker from the Background Type drop down. When you do you'll be given the ability to use a color picker to set the site background color.