Sun/moon (switch mode) button is missing in word 365

I was just upgraded from 32bit Microsoft Office Professional Plus 2016 to 64bit 365 this week. Outlook and OneNote immediately showed dark mode (because it was the system setting). However Word does not appear to support a "dark canvas". I've tried toggling from System to "Black" and checked and unchecked "Disable dark mode" between exits. Still I can only "simulate it" by selecting the dark page background. I also note Excel doesn't appear to even have any support for a dark canvas. My guess is some old Office setting from years back has been copied forward from machine to machine and is now gumming up this option somehow? Account Screen says "Microsoft 365 Apps for Enterprise" Version 2102 Build 13801.21004

The Black theme appears to be working as expected here (Word 2021). I haven't tested 365 after the most recent update.

Sometimes, you have to select and clear the "Disable dark mode" a couple of times and restart Word in between.

It's still not working. I have 50 identical builds, with identical Office 365. 20% of them do not show Dark mode. I can turn Dark on but it does not change the page colour. This is a HUGE issue for my users.

There must be a Reg key not being changed. Does anyone know which Reg key to change to Enact Dark mode properly?

I was just upgraded from 32bit Microsoft Office Professional Plus 2016 to 64bit 365 this week. Outlook and OneNote immediately showed dark mode (because it was the system setting). However Word does not appear to support a "dark canvas". I've tried toggling from System to "Black" and checked and unchecked "Disable dark mode" between exits. Still I can only "simulate it" by selecting the dark page background. I also note Excel doesn't appear to even have any support for a dark canvas. My guess is some old Office setting from years back has been copied forward from machine to machine and is now gumming up this option somehow? Account Screen says "Microsoft 365 Apps for Enterprise" Version 2102 Build 13801.21004

Your version number, 2102, indicates that you're on the Semi-Annual Enterprise Channel. The page at https://docs.microsoft.com/en-us/officeupdates/semi-annual-enterprise-channel doesn't mention Dark mode. The page at https://docs.microsoft.com/en-us/officeupdates/monthly-enterprise-channel says that Dark mode was first released to the Monthly channel in August, in version 2106. I believe that means you'll get Dark mode when you're updated to version 2108, which is currently available only to users on the Semi-Annual Enterprise Channel (Preview). That should happen in a few weeks.

Meanwhile, the first article mentioned above shows that there's an update that was released on 9 November to build 13801.21050.

Found a fix to this. Our standard install with updates did not add the feature. Downloading The updated .exe via the office 365 page and letting it run on the machine fixed it.

I suspect that Older versions of office may have issues with this feature. Rebuilding it using the .exe from Offcie365 website may repair/add it back in.