--also known as the Customize API, is a relatively recent addition to the WordPress ecosystem. In short, it's a framework meant for live-previewing any changes that the user makes to their website. The Customizer's interface allows for the modifcation of various aspects of the theme's and website's appearance, starting from the name of the site, the tagline, all the way to the colors used, layouts, widgets, menus, and more. We talked about the Customizer from the user's point of view back in Chapter 7, Customizing Your Website Appearance/Design.
Now from a developer's perspective, when you're building your own WordPress theme, there are a couple of things that you should do in order to make your theme compatible with the Customizer. Luckily, the starter theme that we're using in this...