atrium_username it is a Drupal feature for managing user name display in Open Atrium.
It's a lightweight alternative to realname module that try to "work" before theme layer (allowing users to plug their proper theme, not only the tao/ginkgo), using the node title of the user profile page to get the display name as follow:
Notification form elements are injected in various forms (node editing, comment etc.) by the notification_team module using the form_alter hook (notifications_team_form_alter). At theme level the module define "delicious" like form subscribing users to nodes.
The hook_form_alter seems the appropriate way to modify the form without affecting the theming:
The Atrium Account (atrium-account) block does not theme the user name as reported by various users. The block subject is set in /profiles/openatrium/modules/atrium_features/atrium/atrium.block.inc: