How can I add a Hover + Text Overlay? (Wells Template)

I was wondering if anyone would know how to add text when hovering over an image in the Wells Template? 

When using the set Gallery page, there is (by default) a very faint overlay applied to the image. I would like to be able to add text to this (in the centre of the image), and possibly increase the opacity of the overlay (as it is barely perceptible). See below for an example of a Wells Gallery page:

https://www.squarespace.com/templates/wells

I don't mind if the CSS only applies to image blocks, and it would be great if the image could be clickable as I am hoping to construct an Index-type page, due to the fact that the Wells template doesn't include that function. 

I'm very much a noob when it comes to CSS, so please bear with me! Any help would be amazing 🙂