list_themes(), theme_enable() and theme_disable() and others have been replaced with a new service

Drupal 8 has introduced a new 'theme_handler' service for managing themes. The procedural functions from Drupal 7 have been deprecated.

Drupal 7:

$themes = list_themes(); // Get a list of available themes.
theme_enable($theme_list); // Enable themes.
theme_disable($theme_list); // Disable themes.
drupal_find_base_themes($theme, $key); // Find base themes for a theme.
system_rebuild_theme_data(); // Rebuild, save, and return data about all currently available themes.

Drupal 8:

$theme_handler = \Drupal::service('theme_handler');

$themes = $theme_handler->listInfo();  // Get a list of available themes.
$theme_handler->enable($theme_list); // Enable themes.
$theme_handler->disable($theme_list); // Disable themes.
$theme_handler->getBaseThemes($themes, $key); // Find base themes for a theme.
$theme_handler->rebuildThemeData(); // Rebuild, save, and return data about all currently available themes.