All - or most of the - theme related settings, are located in the admin's Appearance section.
From this page, it's possible to enable and disable themes. Site builders can even install themes using a URL or by uploading a theme archive (tar tgz gz bz2 zip). From this page, a theme can be set as default user-facing theme using the Set as default button which appears right from the screenshot (this button won't appear for the theme that is currently set as the default theme).
A theme that is installed won't - by default - appear in the front-end, unless it's set as the default theme.
At the bottom of the page, the administration theme can be switched.
Below is an example of Bartik's theme settings page.
Installing a new theme
Drupal scans the
themes directory and looks
*.info.yml to find new themes. Make sure to install themes in the correct directories.
For themes that should be available to all sites.
For themes that should be available on a specific site (on a multisite installation).
@todo: Change location of this part:
- The default settings and config are changed to be fast and safe production values. In a default Drupal 8 installation, CSS and JS aggregation is turned on.