.main {display: grid; justify-content: center;  align-items: center;}

.mybox {height:300px; width:500px; background-color:#D3E8FA; box-shadow: rgba(0, 0, 0, 0.25) 0px 54px 55px, rgba(0, 0, 0, 0.12) 0px -12px 30px, rgba(0, 0, 0, 0.12) 0px 4px 6px, rgba(0, 0, 0, 0.17) 0px 12px 13px, rgba(0, 0, 0, 0.09) 0px -3px 5px;}
.inhaltzentriert {margin-top:100px; display: flex; align-items: center; justify-content: center;}