The final option is to simply create a semi-transparent.png image of the desired color in your preferred image editor, set the background-image property to the URL of the image, and you won't have to worry about mucking around with CSS and losing the capability and organization of a nested div structure.
Replacing an image's original backdrop with a transparent one is a common job in web design and picture editing. There are a plethora of conceivable applications for this: eliminating an unpleasant background when sharing images on social media; making things sold online appear more enticing without the background; removing uninteresting background sceneries to later replace them with futuristic backgrounds. There is good news! You don't have to be a graphic designer to make your photographs' backgrounds translucent. PhotoScissors will take care of it for you. Here's how it works:
