Edge Dev 134.0.3124.5 is now available to Insiders and it's coming with new settings for customizing the browser's theme.

But the color schemes are limited as of now. If you want to change the overall appearance of the Microsoft Edge browser, access the Customize appearance section under the Settings Page.

The Microsoft Edge Beta branch now ... theme includes setting the generated image on the Edge new tab page and applying the image's dominant color to the browser frame. Admins can control ...