This theme utilizes the built-in WordPress Customizer. The Customizer is a framework that gives you live-previews as you make changes to any of the existing options and settings that exists in the customizer. These can be related to WordPress and/or the theme.
It provides a consistent interface for users to customize various aspects of their theme and their site, from colors and layouts to widgets, menus, and more. Themes and plugins alike can add custom options to the Customizer, so when you navigate to it, it may look slightly different.
To access the WordPress Customizer, log in to the WordPress administration area and navigate to
Appearance > Customize and you will see something like this:
When you open any of the tabbed sections, you will find a variety of settings from sliders, text fields, checkboxes, colour selectors, and more.
As you make changes to most, if not all, of the settings, you should see a live preview as you make the changes happen. You see them in the preview window, but they will not show on the front of your website until you click the Publish button at the top of the options panel.