Are you referring to the three widgets at the bottom, underneath the slider? Unfortunately Themify doesn’t make it very easy to target that section, but try adding this CSS to your custom CSS:
#themify_builder_content-104 .themify_builder_row:nth-child(2) {
background-color: rgba(200, 200, 200, .7);
padding: 20px;
height: auto;
}
The value for the background-color property function, rgba, are Red, Green, Blue, and Opacity. For the three colors, the possible values run from 0 through 255. If you want black, it would be 0, 0, 0. If you want white, it would be 255, 255, 255. Right now, I’ve coded a light gray. For Opacity, a value of 0 is completely transparent, a value of 1 is completely opaque. You can use this tool to figure out what values you want.
The padding property adds some spacing around the widgets.