Microsoft is making some improvements to the dark mode in the Windows 10 Mail and Calendar apps, which means that it will now apply to more elements of the interface. This includes the message body and composer in the Mail app, and the event creation interface in the Calendar.
In addition to applying the dark theme more widely, Microsoft is also making it easier to toggle between the light and dark themes. A toggle button will sit alongside other actions, such as replying to a message, so it's easier to switch on the fly.
WindowsBlogItalia reports that the update is now rolling out to Insiders in the Fast Ring or Skip Ahead, but the change doesn't seem to have shown up on our test machines. Microsoft has locked down the inbox apps in preparation for the general public release of Windows 10 19H1, so you'll likely need to be on the Skip Ahead ring to get the new look.
Microsoft has been making small changes to the UI of the apps over time, introducing density elements last year, and it's also been working on an improved design for the sidebar. The latter was spotted a while back, but it's never been fully rolled out to Insiders or the general public.