Computer Configuration > Administrative Templates > Microsoft Office 2016 (Machine) > Global Options > Customize
Machine
HKLM\software\policies\microsoft\office\16.0\common
This policy setting allows you to select the user interface (UI) theme used by Office, if the user has not already selected an Office theme. Note: This setting only applies to Version 1903 or later of Office. If you enable this policy setting, you may choose the Office theme used in cases where the user has not selected an Office theme themselves. If you disable or do not configure this policy setting, Office will use the Colorful theme in cases where the user has not selected an Office theme themselves. Regardless of how you configure this policy setting, users can change their Office theme by going to File > Account > Office Theme (or, in Outlook, by going to File > Office Account > Office Theme). Note: This policy setting takes precedence over the “Default Office theme” policy setting located under User Configuration.