Steps to Reproduce
In an instance running UI16, navigate to System UI > UI Macros.
Find and select the ui_theme_changer UI macro.
Deselect the Active field and click Update.
Despite this setting, the theme picker is still active.
To prevent users from changing their themes:
Access all the themes available for UI16 (Concourse) by going to http://<instance name>/sys_ui_theme_list.do?sysparm_query=device%3Dconcourse.
Set all of the themes to active = false.
Related Problem: PRB658866