<style>
body {
    background-color: lightblue;
}

.border {
    position: absolute;
    width: 200px;
    height: 200px;
    margin: 100px;

    border: 10px solid rgba(0,0,0,0.25);
    border-radius: 10px;
    background-clip: content-box;
    background-color: lightgreen;
}
</style>
<div class="border"></div>