How to Make Background Image in Beaver Builder Row 20% Visible

Question

I am creating a Beaver Builder Wordpress site and cannot figure out how to make a row's background image semi-transparent. I have the Theme Customize -> Content settings to a background color - no issues.

In specific row modules (i.e. containing only a text box) I'd like to add an image to the row background, but then make that image 20% opaque. I know that in CSS this should be possible, but as a new page builder I don't know WHERE to add this code. It seems like if I add an opacity=20% code to the Customize -> Additional CSS would it not apply to the entire site?

If anyone could lead me to the right place and the specific code to add I'd be most appreciative.

Thanks.