Have you noticed in Youtube there is a light switch for Earth Hour?

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Hi there! Some minutes ago a found something odd on youtube. I was watching a video and near the title a notice a sort of "light switch" icon. So I clicked on it and the "light" went off in Youtube . I think Youtube has added this option for today that is the Earth Hour day. It's to save the energy when you turn off the light. I wonder if this option will be only for one day or not. Have you tried it? It's funny!