Theme Switcher Rules
https://www.drupal.org/project/theme_switcher
D8,9,10,11
5191 sites
Switch Page Theme
https://www.drupal.org/project/switch_page_theme
D8,9,10
2149 sites
Switch Theme
https://www.drupal.org/project/switch_theme
D9,10,11
14 sites
Theme Switcher By User
https://www.drupal.org/project/tsbu
D9,10,11
21 sites
Simple Theme Switch
https://www.drupal.org/project/simple_theme_switch
D10+
2 sites
Theme Per User
https://www.drupal.org/project/theme_per_user
D8,9,10
39 sites