IWP only supports the classic theme, so you will have to change the theme to classic. In edit mode, under Layouts, select change theme and change to the classic theme.
You would have to set a field with the user name when they login, and clear it when they logout. If a second user logins and the name is already in the field (in use) then you would exit this second user. This would be done in a startup script using the onFirstWindowOpen script trigger.
IF you like that theme and want to keep it for non web users, use Duplicate Layout to generate a copy of the layout, then change the theme for one of the two layouts. A script that performs on start up can identify the user and switch to the layout that they need.
There is a potential complication with S Chamblee's suggestion on managing user log ins: If a user logs in and then experiences a crash or is disconnected from the database, the system will show them as logged in and they will be locked out until you edit that record for them to allow them back into the database.