With the latest Insider Update Version 1908 (Build 11929.20182) the update was already rolled out. Of course, there was already a Dark Mode available in the settings. As you can see, now it is also covering the reading pane of eMails.
You can also use the Sun/Moon icon on the reading pane to switch only this area to dark or white. This will help you to check how the text will look like on black/white background (as you can see in Screenshot on left side).
I really prefer using the Dark Mode. Also my Windows 10 as per default the Dark Mode enabled for all applications and as well for the File Explorer. If you want to learn more about Microsoft’s dark mode concept take a look at this Medium Article or to this Video.
Location Suggestions for Events/Meetings
Another really cool feature which is rolling out is Location Suggestion for Meetings in Outlook. This is really nice because you can directly see available meeting rooms or can search for locations for meetings like you would do on Bing Maps.
I am really looking forward to more usefull improvements and features like this. This makes planning much easier. For those who would like to use Cortana as a scheduling assistant for meetings, check out the article of Calendar.help.