Why don't you change the theme in the UI. Yes I know it's deeply nested in the options but it seems that there are more steps to change the theme-settings than simply edit a key in the registry. You know evene MS want's to abondon settings in the registry.
I tried install Office 365 ProPlus > actived Black Theme > Uninstall Office 365 by default Windows Uninstaller > Install Office 2016 ProPlus and Black Theme still available but Office Theme name is blank. Unstill I change Office Theme, I lost Black Theme again.
This works every time, but needs to be customized for each Office application you open, for example Word, Excel, Powerpoint, etc. I have a folder on my C drive called Customizations so these files are written for that specific folder. You would need to change the path if you put these three files in a different folder. Here are the three files 1) blacktheme.cmd Code: regedit.exe /S C:\Customizations\OfficeBlackTheme\blacktheme.reg"C:\Program Files\Microsoft Office\root\Office16\WINWORD.EXE" 2)blacktheme.reg Code: Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common] "UI Theme"=dword:00000004 "PendingUITheme"=- 3)blacktheme.vbs Code: Dim WinScriptHost Set WinScriptHost = CreateObject("WScript.Shell") WinScriptHost.Run Chr(34) & "C:\Customizations\OfficeBlackTheme\blacktheme.cmd" & Chr(34), 0 Set WinScriptHost = Nothing Change the path of the shortcut in each program to go to the blacktheme.vbs, and you are good.