linear-gradient 颜色渐变

/* 渐变轴为45度，从蓝色渐变到红色 */
linear-gradient(45deg, blue, red);

  &::before {
    @include size(100%, 4px);
    position: absolute;
    left: 0;
    bottom: 0;
    background: linear-gradient(-26deg, #74C7FC, #699BF9);
  }