如何改变css伪元素的样式

selector:pseudo-element {property:value;}

selector.class:pseudo-element {property:value;}

.content {

    width: 100px;

    height: 100px;

    margin: 0 auto;

    background: black;

}

.content::before {

    content: "";

    width: 20px;

    height: 20px;

    position: absolute;

    background: blue;

}