MediaWiki:Gadget-BackToTop.css

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a.back-to-top {
  display: none;
  width: 60px;
  height: 60px;
  text-indent: -9999px;
  position: fixed;
  z-index: 999;
  right: 20px;
  bottom: 20px;
  background-color: darkred;
  background-image: url(data:image/gif;base64,iVBORw0KGgoAAAANSUhEUgAAAB4AAAAeCAYAAAA7MK6iAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAM1JREFUeNrs12ELgyAQBuDcPx9OxIlrErGfeysQBs4r7zwJohfuo/dEqFcKAIYjchsOygVTcl9Ks1evm4tRGn55cHpwUAP/Mb1hC3hsL9jBfpw07KE+XgoOQE9ohV/Az7iUosIqLWzNG8MxNIJcYgkvoVNlw0+qmkw5nsNzZaM5NVLENSjMeXrKW2LDEdkstfuCBW8ej8qTQIZHwtUapODAmGC+FfbAm9drPbmwa0CxibYLWwG0NMM3YSOI5l8tKKw7oMXe6vqTOD38FWAAl8JSohyMtPcAAAAASUVORK5CYII=);
background-repeat: no-repeat;
  background-position: 43%;
  background-position-x: center;
  border-radius: 30px;
}
a:hover.back-to-top {
  background-color: #000;
}