The Network Admin Themes Screen controls which themes are available to Site administrators in their Administration > Appearance > Themes Screen. Network Administrators must install the themes in the wp-content/themes folder for the themes to be available, and Network Enabled in this screen.
This table lists all of your installed Themes by row. Themes are displayed alphabetically by the folder name used for storing that theme in the wp-contents/themes folder.
The table of themes contains the following columns:
For more detail about installing Themes, refer Install Themes article.
The built-in Theme Editor allows you to view or change any code kept in a Theme Template or Stylesheet files. Select Network Admin > Themes > Edit and select Theme from Select theme to edit box and click Select button, or Click Edit link under the Theme from this Network Admin Themes Screen.
For more detail about Theme Editor, refer Appearance Editor Screen article.
When new sites are created via Administration > Network Admin > Sites, the WordPress Twenty Fifteen theme will be set as the active (default) theme. If a different active (default) theme is desired, edit the wp-config.php and add the WP_DEFAULT_THEME constant definition to the file. For example to make the WordPress Classic Theme the default theme put this in wp-config.php:
If you do remove the WordPress Twenty Fifteen theme then you need to define a new default theme with the WP_DEFAULT_THEME constant.
Disabling a theme in the Network Admin Themes Screen does not prevent that theme being used by a site. It only prevents the theme being listed in the available themes list when selecting a new/different theme.