That is the new Yandex minimalistic browser software from Yandex, and not everything is done yet. The gray lamp icon must be visible in the URL box. If you see not the lamp button in the URL box, then you are using the alpha browser version from Yandex. And it’s possible that this feature not ready to display the extension icon in the URL box. If you want to try the Yandex.browser alpha go to the official Yandex website.
The Yandex browser is based on Google’s open source Chromium project, which also powers Chrome and Opera. And it’s surprise work well and even fast, quite stable, and standards compliant. Also there is a number of intelligent and very useful little tweaks. One of them is the way it handles tabs. Open several tabs from the same site, and Yandex will group them all together in the tab strip.
Turn Off the Lights works well on the stable Yandex browser that can be downloaded on this web page: http://browser.yandex.com And on the Yandex extension store, you can download the Turn Off the Lights extension. If you open now a web page such as YouTube or Vimeo you see the gray lamp button in the URL box. Click on it and it dims the web page and highlight only the video player.