Microsoft has added a dark theme option to its ongoing beta web version of Skype. It's just the latest app from the company to offer such a dark theme.
The new update to the browser version of Microsoft's VoIP service (via MSPU) was quietly added a few days ago and brings the version number up to 1.61.68. It's now available in the web app's Settings menu under the new "Personalization" option. Microsoft added a dark theme to the Skype Preview for Windows 10 in May, and recently added the same option to the Windows 10 Maps app, along with the MSN Sports and Money apps.
Microsoft first announced the beta web browser version of Skype in 2014, making it available to a select number of testers before opening it up to everyone in June 2015. Even though it's been well over a year since the worldwide launch, Microsoft has still not removed the "beta" tag from the Skype web version.