The Anniversary Update makes it available under 'Settings > Personalisation > Colours', where you can 'Choose your app mode'.

Light is chosen automatically, but Dark turns buy office 2016 white backgrounds black. In Tablet Mode, you are able to hide the Taskbar to provide you with more screen space, and also the apps list now appears in full screen.

If your device includes a touchscreen or stylus you'll enjoy the improvements to Windows Ink too, which you can use to highlight and delete words in Office and draw distances in Maps. You can scribble notes, and cheap windows 7 can recognise what you're writing ¡§C for example, should you write a task for later, Windows can change it into a reminder.