how to add a shadow effect featured images in the archives section

Hi there,

Try this CSS:

.post-image img {
    -webkit-box-shadow: 10px 10px 5px 0px rgba(0,0,0,0.75);
    -moz-box-shadow: 10px 10px 5px 0px rgba(0,0,0,0.75);
    box-shadow: 10px 10px 5px 0px rgba(0,0,0,0.75);
}