Position title over image and show only on hover.

envira-title-over-image-hover.css

/* Position title and hide it by default */
.envira-gallery-item-inner .title {
   opacity: 0;
   position: absolute;
   top: 50%;
   left: 50%;
   transform: translate(-50%,-50%);
   
   /* Optional: add transitions so the title smoothly fades in/out on hover */
   -webkit-transition: opacity 0.5s ease;
   -moz-transition: opacity 0.5s ease;
   -o-transition: opacity 0.5s ease;
   transition: opacity 0.5s ease;
}

/* Show title on hover */
.envira-gallery-item-inner:hover .title { opacity: 1; }