A Shopify theme is the layout, look and feel, and aesthetics of your website. This means the layout and design of your header, footer, buttons, the way your categories look, how your product detail pages look, and even some functionality like an Instagram Feed on your home page. The theme is a bunch of code wrapped up in a single file that works only with the Shopify platform. All Shopify themes can be edited with the Shopify theme editor that all stores come up. However, not all theme developers have added many ways to edit the layouts, colors, fonts, and extra section areas. While other developers have added a lot of functionality for you to edit and change many aspects of the theme on your own without a developer.
You may be wondering how the Shopify themes look on mobile. Just to clear up a straightforward question, "Is this theme mobile-friendly," and the answer is YES! If the theme is in the Shopify theme store, then it is "Responsive," which is the terminology used for Mobile-Friendly. You can also demo the mobile version by merely making your browser window smaller, and you will see the responsive design automatically resize as you make your browser screen more narrow. Or if you're in the Shopify Themes demo, then you can click the icons at the top to see Desktop vs. Mobile.
Many people think because they see a shoe company or electronics company in the demo of the theme, they have to pick a different theme. Keep in mind that you're looking at the layout, styling, not the content. For example, we commonly have apparel companies that think because the theme shows bicycles, they need to pick a different theme that shows apparel. It's just a layout, all the colors, content, banners, and many layouts can be changed to make the theme match your brand.
When you are looking at a theme, you will see a style selection. Usually, there are 3-4 styles on the demo page in Shopify Theme Store. What most people don't know is, you're not buying a style, meaning it is not 3-4 different themes. The styles are simply the different ways you can mix and match all the settings in the theme to make it look that way. The theme has many options, settings, layouts, fonts, colors, section options, etc. So the style is just a single example of what you could do. You get all styles when purchasing that specific theme.
All themes now have sections. This means on the home page you can rearrange the order or each section, add new sections, hide sections while your working on them, and more. So when you're looking at the demo, just keep in mind that you're not stuck with the order of each section, you can add, remove, and rearrange anytime you like. It's very easy!