/* 设置了从上到下的渐变黑色 */
#theme-matery .header-cover::before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background:  linear-gradient(to bottom, rgba(0,0,0,0.5) 0%, rgba(0,0,0,0.2) 10%, rgba(0,0,0,0) 25%, rgba(0,0,0,0.2) 75%, rgba(0,0,0,0.5) 100%);
}