The hover box seems to be contained and styled via the .grid-item .overbox rules.

This starts on line 1199 of your style.css when inspecting in Chrome.

.grid-item .overbox {
	position: absolute;
	top: 50%;
	width: 100%;
	text-align: center;
	line-height: 1;
	padding: 0 1em;
}

if you add transform:translateY(-50%); to that rule it will vertically align the box better.

The title itself has styling on .grid-item .title { and should be found on line 1261 of your style.css. I reduced the font size to 1.5em and it started to look a lot better.

Finally the green colour is applied to the anchor .grid-item .overbox h2 a { and can be found on line 1228.

I hope this helps

All the best