By default, Drupal 8 ships with three themes. As part of the standard installation profile, Drupal will install and configure Bartik, Seven, and Stark themes. Each of these themes serves a specific function in the workflow. Let's look at them in more detail.
Bartik is considered the default theme in Drupal and is familiar to most as it has been part of the Drupal ecosystem for quite a while now. We can think of Bartik as the frontend theme or what we see when we first install Drupal. The Bartik theme is what you will visually see when you are not navigating within the Drupal administrative screens.
Seven is the default admin theme, and it provides a clean separation between the frontend and backend of Drupal. This is great as it will always allow us to navigate through the administrative areas if our default theme generates any errors that may cause a blank white screen while theming.
Stark is an intentionally plain theme with no styling at all to help demonstrate the default HTML and CSS that Drupal will output and is great for learning how to build a custom theme.
Wait, this is the fourth theme! Actually, Classy is a base theme that both Bartik and Seven use that provides both with clean well-documented markup and CSS classes. Classy is hidden from the Appearance admin screen by default, and we will learn more about Classy as a base theme and how to use it within our own themes later in Chapter 4, Getting Started – Creating Themes.