The Windows 10 Mail and Calendar apps are getting an update for Insiders, and it seems to improve the dark mode by applying it to more elements of the UI, such as the message composer.
Windows 10 mail RSS
Microsoft is rolling back its decision to force users of the Windows 10 Mail app to open links in its own Edge browser, and it will provide users with the option to use a different one.
Microsoft has updated the Windows 10 Mail app for Insiders in the Fast Ring, introducing inking capabilities in the composer, but the feature doesn't seem to be available for everyone yet.