oh, I checked the CSS of bolderless press and it is causing the hover outline
.mg-image-tiles.borderless .mg-thumbs ul li div:hover {
box-shadow: 0px 0px 5px #808080 !important;
-moz-box-shadow: 0px 0px 5px #808080 !important;
-webkit-box-shadow: 0px 0px 5px #808080 !important;
}
Which could be overridden with
.mg-image-tiles.borderless .mg-thumbs ul li div:hover {
box-shadow: none !important;
-moz-box-shadow: none !important;
-webkit-box-shadow: none !important;
}