Some weeks ago, Microsoft released a new browser based on Chromium.
At the moment you can use two main released channel: Dev and Canary.

In the future it will be possible to download and use the Beta Channel.

At the moment you can enable the dark theme in both channels.
It's very simple:

• Open a new tab on Edge and go here: edge://flags/#edge-follow-os-theme
• Select "Enabled" in the dropdown
• From your desktop, go to Personalization -> Colors and change the theme to dark on Windows

Now the Edge theme reflects your Windows theme.