The Customizer is a framework for live-previewing any changes as you make them WordPress. It provides a simple and consistent interface for users to customize various aspects of their theme, website, colours, layouts, widgets, menus, and more. Themes and plugins alike can add custom options to the Customizer.
Every free and premium theme we have uses the WordPress Customizer to make it easier for you to personalize your theme and website. Each theme will also differ in what options and settings are included.
To access the WordPress Customizer, go to Appearance > Customize.
If you have the Welcome screen showing on the dashboard home page, you can also access it from the big Customize Your Site button: