Dark mode also called black mode or light-on-dark mode is a setting in which text and visuals are presented in a light-colored scheme on a dark (usually black) background.

More and more email clients are supporting dark mode in emails:

Besides our normal preview mode, we provide a dark mode as well to let our customers check their emails how they will look like in Outlook’s dark mode implementation.

You can switch from the normal preview mode to the dark mode by clicking the Light mode button in the top right-hand corner and selecting Dark mode (Outlook) from the dropdown list.

This is what your email would look like in dark mode.

We have a tutorial on how to design emails in dark mode here.

Here you can read more about the dark mode feature.