Hello,
The default overlay is an image overlay here:
Template.CSS - Line 2067
.image_title {
background: url("../images/repeatexture.png") repeat fixed 0 0 transparent;
If your not using this image overlay it will default to the module overlay which is here:
s5_horizontal_accordion2.css - line 26
.image_title {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0.5);
bottom: 0;
left: 0;
position: absolute;
text-align: center;
width: 100%;
}