browsers simply give your pages white backgrounds if you don't override that default. Add a short CSS class to an HTML Web page to use animated and stationary GIF images as the page's background.

Cascading Style Sheets (CSS) give ... images and color to the background of any element on a Web page. It opens up a creative toolbox for styling links, bulleted lists, and more. Unlike HTML ...

Your Tumblr theme sets the background ... into the box labeled "Add Custom CSS": Replace "yellow" with the specific color that you want to apply or with the color's HTML code.