Like the OPAC, this uses Template Toolkit for the HTML and JavaScript:

• Template Files: Open-ILS/src/templates/ except the staff/ and opac/ subdirectories.
• JavaScript Files: Open-ILS/web/js/dojo

As the name implies, it used the Dojo toolkit, but a very old version of it (1.3): https://dojotoolkit.org/reference-guide/1.9/releasenotes/1.3.html

Evergreen used to have a XUL staff client (desktop application), for which pieces still exist under Open-ILS/xul/.

• Historical explanation