Nope I hadn’t changed the background colour…I’ve fixed it now tho. I changed the background colour below to #000000 and it seems to have done the trick!
.wp-caption {
border: 1px solid #ddd;
text-align: center;
background-color: #f3f3f3;
padding-top: 4px;
margin: 10px;
/* optional rounded corners for browsers that support it */
-moz-border-radius: 3px;
-khtml-border-radius: 3px;
-webkit-border-radius: 3px;
border-radius: 3px;