Hi there,
In Customizer > Additional CSS look for this:
.wpsp-card .wpsp-content-wrap {
position: absolute;
bottom: 0;
left: 0;
right: 0;
padding: 5% 8%;
-webkit-box-sizing: border-box;
box-sizing: border-box;
background-color: rgba(0, 0, 0, 0.35);
background: -webkit-gradient(linear, left bottom, left top, color-stop(30%, rgba(80, 50, 50, 0.5)), to(rgba(0, 0, 0, 0)));
background: linear-gradient(0deg, rgba(80, 50, 50, 0.5) 30%, rgba(0, 0, 0, 0) 100%);
pointer-events: none;
}
This adds a gradient behind the text of the Post Grid.
These two lines add a gradient fill behind the text:
background: -webkit-gradient(linear, left bottom, left top, color-stop(30%, rgba(80, 50, 50, 0.5)), to(rgba(0, 0, 0, 0)));
background: linear-gradient(0deg, rgba(80, 50, 50, 0.5) 30%, rgba(0, 0, 0, 0) 100%);
Change the rgba values to suit