Your website theme (also commonly referred to as a template) is what defines the overall design of your website, such as the colors, fonts, background images, and other design elements.

Simple theme changes can be made under the Design Options area, but to really customize the look of a website, you need to visit the Advanced HTML/CSS Theme Editor.

Enter the Theme Editor by clicking on Edit HTML/CSS in the Design Tab:
You'll be presented with our Theme Editor, allowing full HTML & CSS flexibility