How can I change the blue background color that is outside of the zen cart?
Find the stylesheet.css that goes with your website. The stylesheet is under the includes/templates/YOUR TEMPLATE NAME HERE/css folder
Search for the word "body". In most templates, this is usually the first line of the stylesheet.
Under that section, you will see background-color: #e5edf5;
Change the six digit hex code to the color you want for your body background.