Applying Black Overlay to Homepage Banner Image

I want to add a black overlay to the homepage banner image on https://octagon-mandolin-lmws.squarespace.com/ for a more striking visual effect. Looking for the CSS to achieve this.

You don't need CSS for this, for each section colour theme you can set the background overlay colour (in Styles -> Color, and pick the relevant theme). You can then adjust the overlay opacity for each section in the section settings.