I installed Office 2016 as an upgrade to 2013. Put the color scheme on grey (because annoyingly the awesome Black went away sometime in the preview.)
Now I have a weird issue where every time I close all of the 2016 applications and then reopen one of the applications, the theme has changed from grey to colorful. This applies to reboots or just closing them all and then later opening one back up. Oddly
as well today (10/5/2015) I walked away from the computer to talk to a co-worker in the next office. When I came back, it was on colorful again. However that is the only time it has changed itself while open.
It seems to work correctly when it is set to Grey. So for instance, say I open Outlook and change the theme to Grey. When I open Word or Excel, they are also Grey as they should be.
So I'm just throwing that out there as a potential bug