How do I let users with a certain role add nodes but use the front end theme (not the admin theme)
Role Theme Switcher seems to be what you need:

Useful Drupal module to assign separate themes for different roles (including anonymous) in your system.
Often you want to separate Drupal administrative interface from frontend. You can do this assigning appropriate theme for you as user. What to do if you want to show different theme for non-authorized user?