https://www.theverge.com/2019/3/5/18251387/microsoft-windows-10-mail-dark-mode-feature

Microsoft is introducing a more complete dark mode for its Mail app in Windows 10 today. The software maker has started rolling out a new update that includes a dark mode for the email portion of the app. Previously, the app would show a white background in emails instead of a black one. Microsoft has also added a toggle to switch between dark and light backgrounds in the body of emails, as some email content will be obscured with dark mode enabled.

The toggle and full dark mode are similar to Microsoft’s Outlook.com service. Microsoft introduced a dark mode for Outlook.com back in July, and the Mail app of Windows 10 now mostly matches it. Microsoft’s calendar portion of the Mail and Calendar app for Windows 10 remains the same and has always supported a full dark mode. You can download the latest Mail app for Windows 10 from the Microsoft Store.