If you’re working in the default “Pages” view, where pages are shown with a background-style border, it will change the color of your page. Not the background behind that.

The background color is changed. From the Edit menu, you can also directly access the HTML code by clicking "Edit HTML" at the top of the page to open the HTML Editing window in the left pane and ...

As you may or may not know, you can customize the background of a Terminal window in macOS. You can use either a color or an image, and you can also set a transparency level for the selected color ...