Hi Kevin,
The CSS in the docs works, but it requires gallery ID which is not very practical for me as I have a lot of galleries. However, I managed to get the desired overlay affect that automatically applies to all gallery using this CSS:
.envira-gallery-item-inner a { display: block; border-bottom: 0 none; transition: background 0.3s ease-out !important; }
.envira-gallery-item-inner a:hover { background: #012558 !important; }
.envira-gallery-item-inner a:hover:after { position: absolute; content:'+'; color: #ffffff; font-size: 42px; top: 30%; left: 0; right: 0; width: auto; margin: 0 auto; text-align: center; }
.envira-gallery-wrap .envira-gallery-public.envira-gallery-css-animations .envira-gallery-item img { transition: opacity 0.3s ease-out !important; }
.envira-gallery-wrap .envira-gallery-public.envira-gallery-css-animations .envira-gallery-item .envira-gallery-item-inner:hover img { opacity: 0.5 !important; }
Anyway thanks for the tip, Kevin!