One of the things that annoy dark theme lovers on Windows 10 is the poor implementation of it in the operating system. Its competitor Apple despite being late to the party stole the show for better implementation.
Microsoft wants to change that. The company has recently rolled out an update to the Mail and Calendar app and it brought dark theme everywhere in the app. Even the mail content now has a dark theme. Moreover, the dark theme looks much better after the latest update which took the version of the app to version 16005.11231.20142.0.
However, the update is only available to Windows Insiders in the Release Preview ring, meaning if you are a regular user then you need to wait until Microsoft makes this available to the public. Which, by the way, looks sooner than ever before. Otherwise, if you are an Insider registered in the Release Preview ring update your Mail and Calendar app from Microsoft store or you can click here which will directly take you to the update page.