css让层水平垂直居中

    .myDiv{
        position: absolute;
        top: 50%;
        left: 50%;
        transform: translateX(-50%) translateY(-50%)
    }

在父窗体上声明

<div style={{height:'100px',width:"100px",backgroundColor:"ActiveBorder",position:"relative",display:"flex",alignItems:"center",justifyContent:"center"}}>