In 2009, Microsoft launched Windows 7 which supported a new type of theme that updated its images periodically via an RSS feed. They were called dynamic themes and Microsoft released a number of them over the years, including the Sky theme which was first made available fairly recently in August 2013.
However, and with no advanced warning, Microsoft has now stopped offering and supporting their Dynamic Themes. None of them are available for download at Microsoft's Windows website, and clicking on the individual links to each of those themes now shows a message that they have been "retired."
WindowsObserver.com contacted Microsoft for comment and got this response back from a spokesperson:
RSS themes were retired from Windows.com due to a need for efficiencies. Customers can maintain themes already downloaded to their PC but won’t receive new images.
While dynamic themes are no longer available from Microsoft, Windows 7 and 8 users can still get a new desktop background delivered to them every day by downloading and installing the company's Bing Desktop program. The application automatically updates the background with the same high resolution image that's put in place each day on the Bing.com homepage.
Source: WindowsObserver.com | Image via Microsoft