I have a role that allows users to create nodes themselves. What is the standard practice way to let them add nodes but not see the administration theme (I am using adminimal), and rather use a different theme on the node/add page.
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?