weseek__growi/src/client/styles/scss/_button.scss

// fill button style
.btn-fill {
  position: relative;
  overflow: hidden;
  color: white;
  text-align: center;
  cursor: pointer;
  background-color: rgba(lighten(black, 20%), 0.4);
  border: none;

  .btn-label {
    position: relative;
    z-index: 1;
    color: white;
    text-decoration: none;
  }

  .btn-label-text {
    position: relative;
    z-index: 1;
    color: white;
    text-decoration: none;
  }

  // effect
  .eff {
    position: absolute;
    top: -50px;
    left: 0px;
    z-index: 0;
    width: 100%;
    height: 100%;
    transition: all 0.5s ease;
  }

  &:hover {
    .eff {
      top: 0;
    }
  }
}