Outlook email has been dark for ages under Windows 10 Mobile, of course, at least if you set the system theme dark. Yet even then we have email contents turning up 'white' - yet outlook.com's new beta now includes a full on, genuine all-dark mode. OK, so it's nothing to do with Mobile, but it's to do with your Microsoft account and mirroring what you see on the phone, so...

From Microsoft:

Why is Dark Mode not a theme?
When we designed Dark Mode, we debated having it exist in Outlook.com as a theme. We quickly concluded that the way most people would expect themes to work and the way Dark Mode works were fundamentally different. Also, we imagine that there would be a dark experience for all available themes in the future, so having Dark Mode be a separate toggle would facilitate that. Plus, this is consistent with many other applications that have a dark mode.

Will Dark Mode support themes?
Dark Mode only supports the default blue theme right now.

Why does my reading pane have a dark background for email messages I receive?
One of the most crucial principles we had when designing Dark Mode was to minimize the amount of eye strain that people felt. Many email clients on the web today advertise a dark mode, but we learned from interviews with others and our own usage that having the reading pane be on-light while the rest of the interface was left on-dark often made the experience worse than if the full screen were left on-light.

For 2017’s Halloween Theme, we experimented with placing a light gray background in the reading pane. While that resulted in a decrease in eye strain, the design wasn’t ultimately one that we liked for various reasons. We iterated on this problem a couple more times before landing on the solution that you now see, where email messages that you receive are recolored in a secret way that preserves the readability of the original message and preserves the intent of the original sender.

How do I see the original formatting?
You can see any email message that you receive in the original formatting by using the “Turn on the lights” button.

Well worth noting anyway, especially if you access your Microsoft account email through a browser tab and not in an application. You do need to be using the 'beta' site - a toggle top right if you need it. And then just click on the Settings cog and toggle 'dark' on and off, to try it out:

'Dark mode' is an easy toggle - note that you can ignore the 'theme' selection above the toggle - it's overriden by the 'dark' choice.

One click and everything's black!

Well, almost everything, graphics are usually in their original colours, but others are applied correctly with dark background according to their transparency properties.

Other graphics fare less well, as seen here. It's early days though and the styling of dark mode pages will improve. Maybe we'll even get dark mode theme support too?