How can I remove 'Dark Mode' from Ms Office apps without disabling it across macOS?
To disable Dark Mode for MS Office only, you need to run a terminal command for each of the MS Office apps in question.

For example, if you wish to disable dark mode in MS Excel, follow these steps:


Quit Excel if it’s running
Launch Terminal (usually found in the Applications/Utilities folder)
Enter (or copy and paste) the following command: defaults write com.microsoft.Excel NSRequiresAquaSystemAppearance -bool yes
Now press return
Enter (or copy and paste) the following command:  killall cfprefsd
Now press return


Now when you launch MS Excel it should appear as it it normally would without Dark Mode enabled.

NOTES: 


To do this with the other MS Office apps, just change the command at Step 3 so that it’s referencing the app in question.
Each of the app names within the command must start with an uppercase letter (note the uppercase 'E' for Excel in the command at Step 3.
For PowerPoint, the app name must be spelt as follows: Powerpoint (i.e. only the first P is in uppercase, not both Ps)
To revert back to Dark Mode, follow the same steps but end the command at Step 3 with no instead of yes.