Mozilla has announced plans to deprecate complete themes and lightweight themes. Lightweight themes only modify the browser frame’s background, along with a few colors and buttons here and there, but nothing major. Complete themes are hard to create and hard to maintain because they break with every new Firefox release. Mozilla says that supporting the technologies that allow complete themes to work is limiting its ability to improve Firefox. The browser maker has announced official plans to drop support for complete themes in the near future, albeit an exact date has not been provided.
Source: https://www.bleepingcomputer.com/news/software/mozillas-plans-for-firefox-themes-in-2017/