Post Image Studiopress Theme Decor

Large images placed as posts on the studiopress Decor theme showes a thumbnail with white background, a number, month and year on top of every photo or image. I like to have it removed. How can we do this.
I have no knowledge of coding, but 2 people already told me to go Style.ccs and replace “display: block; with none;
Itry to do this, but it does not let me make the change as part of the theme. Any advice, please!

.post-photo .post-date {
-moz-box-shadow: 0px 3px 4px rgba(0, 0, 0, 0.20);
-webkit-box-shadow: 0px 3px 4px rgba(0, 0, 0, 0.20);
background-color: #fff;
box-shadow: 0px 3px 4px rgba(0, 0, 0, 0.20);
color: #7d7d7d;
display: block;
font-size: 14px;
padding: 20px 10px;
position: absolute;
right: 20px;
text-align: center;
top: -2px;
width: 100px;
z-index: 999;