Once you've uploaded your theme, it's time to make it your own. While all of your products, collections, pages, and navigation are configured in the Shopify admin, any changes you make to the look and feel of your store happens in the theme editor. Access it by navigating to Online Store > Themes and then click the purple Customize button.
The theme editor is divided into two tabs: Sections and Theme settings.
The Sections tab contains all of the sections available to you in Reach. Essentially, sections are the building blocks of your store, allowing you to assemble pages from customizable, exchangeable components. You can add, remove, and reorder sections, as well as access the customization features for each from the Sections tab. For more information, here is an in-depth blog post all about sections: Shopify Sections: Helping You Build a Better Online Store
Theme settings tab
The Theme settings tab lets you make global changes to the look of your store, including colors, typography, and products, as well as links to your social media accounts.
Theme actions menu
The Theme actions menu at the bottom of the settings bar contains a bunch of handy links.
Edit code provides access to the theme files.
Edit languages lets you both change the language of your theme and Checkout page, and customize the wording that is used throughout your store.
Device view selector
On the right side preview's top bar, there are three buttons that let you toggle between previewing your site look on mobile, desktop, and expanded desktop.