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);
}