As per Google’s tradition on April Fools’ Day prank, Google announced many April Fools’ Prank. One of prank was that Gmail Blue. If you are the one who watched the Gmail Blue video, you see that how Google makes us fool.
Thanks to Shon Dalezman, who made a Chorome Extension which turns you Gmail into Gmail Blue, which you have watched on the video. it’s just a quick CSS hack applied to any detected instance of Gmail.