Microsoft Word has offered an optional dark mode for a while now, providing a better reading and editing experience at night. It has been missing from the online version, but that's finally changing.
Starting today, dark mode in Word is no longer limited to the desktop and mobile apps. Microsoft said that a dark mode in the web app was a top request among Office Insider testers, and now it's finally available. Once it has finished rolling out, the feature will be accessible from a new 'Dark Mode' button in the View tab of the toolbar. Word will also load in dark mode by default if your browser and/or operating system is set to dark mode.
Dark mode switches the entire Word interface to a dark theme, and applies a dark background (and inverted text colors, if necessary) to the document. However, the actual color data of the document isn't changed, just like with dark mode in the desktop apps.
If you don't like dark mode, you can switch it off by clicking the same button. There's also an independent toggle for the style of the document -- if you need to quickly check how a document will look when viewed normally (possibly blinding you temporarily), there's a 'Switch Background' button at the bottom of the screen and the View ribbon. The switch button state is also saved to your browser's cookies, so you don't need to toggle it again for each document you open.
Dark mode is rolling out now to everyone using Word for the web.
Source: Office Insider Blog