This WordPress User Guide Appearance blog post will show you how the appearance section works in the WordPress portal. While the appearance section allows you to choose between different WordPress themes. The theme itself can therefore be managed through the Customiser, Widgets and Menus options. The Customiser option allows you to manage parts of your theme providing you an easy way to change things like colors. With a widgets enabled theme, this screen allows you to drag and drop items into your sidebar or header/footer areas. The widget screen allows you to add built in WordPress functionality like Latest Posts or you can add raw HTML.
wordpress user guide appearance