How do I change the sidebar background color in FIVE template?

I am trying to make the sidebar area a darker color than the main body background, but it is not a design option. What is the custom CSS to make that happen? I don't know the title of that area or how to even find CSS on the template. Custom CSS seems to be the only option.

The site is still in trial mode so I don't have a public page. Screen Capture attached

Can you add a site wide password and share your URL:

https://squarefortytwo.com/squarespace-guides/add-a-site-wide-password-to-your-squarespace-website

You can try this code to Design > Custom CSS (or Website > Website Tools > Custom CSS)

/* Five Sidebar Color */
div[id*="sidebar"] {
    background-color: darkgray;
}