Squarespace 7.1 - Slideshow Image Overlay Color

@skalison 

The following CSS will target the 2nd slide by using the :nth-child() pseudo class. It the :after pseudo element  to add a layer on top of the image and uses the rgba() css function to add background color of black with an opacity of .4 

CSS

[data-section-id="6351e1b422792e0685dc2531"] .slides .slide:nth-child(2) .slide-media-container:after {
  background: rgba(0,0,0, .4); 
  content: '';
  display: block;
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
}

Here's a recording changing the opacity value of the background color: