The Microsoft Edge browser uses its the light theme as default, but you can change this to the dark theme if you prefer the look and feel on Windows 10.
Follow these steps to make the change:
Open Microsoft Edge
Click the Settings and more … (top right) and click Settings
Under Choose a theme change it from Light to Dark, the will then instantly see the Dark theme
To continue browsing, just click anywhere on the background of Edge and the Settings should disappear.