The Office 365 apps (version 2011 build 13426 or later) can now apply your system theme.
As you know, Office applications had the ability to apply specific themes (colorful, dark gray, black or white); well now it can also apply your system theme.
To make your Office applications use the system theme, launch any of your Office application and go to the Account section to select Use system setting under the Theme selection
Your Office theme will automatically switch to the theme selected in your system settings (Settings\Personalization\Themes)
The Colors option – dark/light – also impact your Office theme.