Adding category to grid thumbnails?

i found i nice options for adding a date to grid thumbnails
http://wordpress.org/support/topic/adding-date-to-grid-thumbnails?replies=7

.date-meta {
background-color: #000;
opacity: .5;
filter: alpha(opacity=50);
position: absolute;
padding: 5px;
top: 21px;
right: 20px;
color: #FFF;
}

function pinboard_post_thumbnail() {
if( has_post_thumbnail() ) : ?>

<a href="” rel=”bookmark” title=””>

<?php endif;
}

I try to change the code for adding the category but im to stupid to find it (no php expert)

can someone give me the solution plz?

thx