The background color of a Web page without any customization is white. HTML Web programming language enables you to change the background color as well as add a background image.

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.

(If you don’t see this you need to click Command + T to open the Start page in a tab.) Here you can activate the options offered by Safari to change the start page, including Background Image.