Themes for your online store
A theme is a template that determines the way that your online store looks and feels. Think of the admin as your office and stockroom, and your online store as your sales floor. Your theme is the architecture and interior design of your sales floor. Different themes have different styles and layouts, and offer a different experience for your customers. For example, if you're selling spa products, then you probably want your online store to convey a relaxed and luxurious feel. Alternatively, if you're selling electronics, then your online store might need to have an energetic and sleek look and feel.
Making changes to your theme or switching to a new theme doesn’t affect the other parts of your account. Theme changes only affect the front-end (customer-facing parts) of your store. They don't affect the back-end (what you see in the admin). This allows you to play around with different theme styles and settings without worrying about the rest of the content in your admin.