hexo-theme-ayer/source-src/css/_partial/totop.styl

@keyframes rocket {
  0% {
    transform translateY(0)
    opacity 1;
  }
  45% {
    transform translateY(-3.2rem)
    opacity 0;
  }
  55% {
    transform translateY(3.2rem)
    opacity 0;
  }
  100% {
    transform translateY(0)
    opacity 1;
  }
}

.totop,.todark
  position relative
  display block
  overflow hidden
  cursor pointer
  rounded-circle(3.2rem)
  background-color darken(froth-light,2%)
  line-height 3rem
  opacity .6
  transition(opacity 1s ease)
  &:hover
    opacity 1
  
.totop
  margin-bottom .8rem
  &:hover > i {
    animation rocket .3s ease-in-out
  }