box-shadow: 0 0 2.5vw #237ad4 inset;
background: linear-gradient(#1359df, #1359df) left top,
linear-gradient(#1359df, #1359df) left top,
linear-gradient(#1359df, #1359df) right top,
linear-gradient(#1359df, #1359df) right top,
linear-gradient(#1359df, #1359df) left bottom,
linear-gradient(#1359df, #1359df) left bottom,
linear-gradient(#1359df, #1359df) right bottom,
linear-gradient(#1359df, #1359df) right bottom;
background-repeat: no-repeat;
background-size: 0.1vw 18vw, 1.5vw 0.1vw;
div四个角加边框,并待内边阴影
效果图: