One of the advantages of Drupal is the ability to have multiple themes available to use at any time, and as we discussed earlier, Drupal provides us with three themes to start with. However, it is important to differentiate between installed, uninstalled, and default. We can consider these as the theme's states.
Installed themes are always located in the Installed themes section of the Appearance admin and are available for Drupal to use for either the frontend or backend of the CMS. However, there can only be one theme set as the default at any given time. We can see a list of installed themes, as shown in the following image:
Uninstalled theme(s) are themes that Drupal is aware of within the core themes folder or the custom themes folder but have not been placed into an installed state. One or multiple themes can be present at any time within the Uninstalled theme section, as shown in the following image:
Finally, we will often hear the term default theme being used, so it's important to remember that the default theme is always the current theme being displayed to users when viewing our website as an anonymous or logged out user or when logged in but not within an Administrative section of Drupal. Anytime a theme is set as default, it will always be considered installed as well.