Skype Preview was updated today from version 11.3.119 to 220.127.116.11, adding a dark theme and multiple accounts support.
The update was originally supposed to be made available last week alongside Windows 10 for PCs build 14342, but it was held back. In fact, the whole thing was a bit of a mix-up, with the build releasing around 11 PM ET. The release notes were edited to say that the Skype Preview update would be released the next day, but it didn't actually happen until now.
Here's a look at the new dark theme (without splicing it with the light theme):
The theme in the app will match the theme in your PC's settings, so don't expect to see a separate toggle in the app's settings. As you've probably guessed, you should be on build 14342 to get the app.