Themes are the way to change the look and feel of a website. It can be as simple as changing a few colors, or you can rewrite everything, if your heart desires. Usually, it's something in between.
Themes have four basic components that work together with which you can effectively implement to create custom designs for your website.
HTML Template is based on Liquid, a markup language that is powerful and easy to use. For more information on customizing a website, please view the Theme documentation.