Selling online, as with brick-and-mortar retailing, is more than simply having products to offer to customers. Buyers make purchases based on many things apart from the item itself. Customers want to understand the purpose of the retailer and have confidence that the seller is legitimate, safe, and honest. Corporations have spent billions over the years creating this understanding among their active and potential customers, through logos, design, copy, and service. Creating this understanding is called branding.
For an online store, branding is very important. However, unlike the offline shopper who drives to a store, parks, and enters the store to spend several minutes shopping; an online shopper may click to your online store, spend a few seconds to determine if they wish to remain, and leave to visit another website. Therefore, your store's brand—it's graphics, copy and function—must address the following in a matter of seconds:
Magento gives you, or your client, the functional tools and systems to provide a powerfully rich shopping experience for your customers. It cannot, however, provide the branding aspects relating to design. You have to craft the outward appearance that will communicate to the potential buyer feelings of convenience, product selection, and security.
In this chapter, we will teach you:
In Chapter 2, Managing Products, we discussed the Global-Website-Store, or GWS, methodology. As we have learned, Magento allows you to customize many store aspects at some, or all, of the GWS levels.
The same holds true for themes. You can specify the look and feel of your stores at the Global, Website, or Store View levels (themes can be applied for individual Store Views relating to a store) by assigning a specific theme.
In Magento, a group of related themes is referred to as a design package. Design packages contain files that control various functional elements common among the themes within the package. By default, Magento Community installs one design package with two themes.
Themes within a design package contain the various elements that determine the look and feel of the site: layout files, templates, CSS, images, and JavaScript. Each design package must have at least one theme, but can contain other theme variants. You can include any number of theme variants within a design package and use them, for example, for seasonal purposes (such as holidays or back-to-school).
The following graphic shows the relationship between design packages and themes:

A design package and theme can be specified at the Global, Website, or Store levels.
Most Magento users will use the same design package for a website and all descendant stores. Usually, related stores within a website business share very similar functional elements, as well as similar style features. This is not mandatory; you're free to specify a completely different design package and theme for each store view within your website hierarchy.
A design package and theme can also be specified based on a per browser basis at the global and website level, by providing a User-Agent Exception. In the following example, the string for the Firefox browser is provided:

To be certain about the string used in the User-Agent Header, you can check http://whatsmyua.com/ from the browser in question. This site will provide a very specific string you can use for the Search String field.
Magento 2 is a departure from Magento 1.x in that the skin directory used in Magento 1.x is absent, and the structure has been simplified. All files have been consolidated into one location, stored under the /app directory. Additionally, the layout and template directories from Magento 1.x have moved. Now, every module has its own layout and template directories in which you can access all template, layout, JS, and CSS/Less files.
Let's take a closer look at the new structure of a theme for Magento 2.0. The theme directory will typically include the files and directories visible in the following image:

The theme directory
You may notice that in some default Magento 2.x builds, some themes are found in the vendor/ directory. This indicates that a theme has been added via a composer dependency. We'll cover packaging your theme via composer later in the chapter. While Magento does check the vendor/ directory for themes, you should never add a theme directly to this directory. When creating a new theme, always start in the app/design/frontend/ directory.
In the new theme structure for Magento 2, all of the files related to design are in the preceding layout, under the VendorTheme directory. To clarify this new layout, we've provided a brief description of the role of each of the files and directories here:
/composer.json: This file is not a required file, but you'll use it to describe the dependencies your theme has and metadata for your theme. This file is essentially for use if you intend your theme to be a composer package./etc/view.xml: This file is required for a theme, but only if it doesn't already exist in a parent theme. We'll review parent themes later in this chapter as well. It includes information about product images and thumbnails./i18n: This directory includes CSV translation files./media: The /media directory contains a theme thumbnail, typically a screenshot of the theme that serves as a preview of the theme when it's being installed in the admin. This directory is required./registration.php: This file is required; it registers your theme within the system./theme.xml: Also mandatory, the theme.xml file declares the theme as a system component. It contains information about the theme like the theme name, the parent theme name, and so on./Vendor_Module: This directory is the parent for module specific styles, and the naming convention is important. Magento will use it to determine which module it needs to apply the styles, layouts and templates to. If the module being overridden was the Catalog module, the directory would be Magento_Catalog. It is only required for modules that are being overridden./Vendor_Module/layout: This directory contains files that extend or establish module layouts. Layout files are XML that determines the position of blocks and containers on any given Magento page. We'll go into additional detail around layout files and the elements in them in the customizing themes section of this chapter./Vendor_Module/layout/override/base: Files in this directory specifically override the default layouts for the module in question./Vendor_Module/layout/override/parent_theme: Layout files in this directory specifically override the parent theme layouts for the module specified./Vendor_Module/templates: This directory contains template files that override either default or parent templates in place for this module. If there are custom templates for this module, they're stored in this directory as well. We'll review template files in greater detail in the customizing themes section./web: This directory contains static files that can be loaded directly from the frontend. For example, if you wanted to reference a specific image, you could include it here, and you would be able to refer to it in a template file./web/css/source: This is the location where Less CSS configuration files for the theme are stored. Less is a CSS pre-processor that allows for variables and functions, making CSS for the theme more maintainable and extensible./web/css/source/lib: This directory contains files that extend or establish module layouts. Layout files are XML that determine the position of blocks and containers on any given Magento page. We'll go into additional detail around layout files and the elements in them in the using themes to create your own look and feel section of this chapter./web/fonts: This contains fonts specific to this theme. Everything in the web directory is static, so this folder is simply here for organizational purposes./web/images: This directory stores images specific to this theme./web/js: This location contains JavaScript specific to this theme. We'll talk more about how to insert JavaScript into a template in the using themes to create your own look and feel section.