Quark: where change overlay-dark?

Forgive the basic question, but where do I darken the "overlay-dark" class?

That is, I want to change the CSS from

.overlay-dark .image-overlay{background:rgba(0,0,0,.4)

to

.overlay-dark .image-overlay{background:rgba(0,0,0,.7)

But I don't know which .css file to edit.

Using Chrome's developer tools, it looks like it is pulling from theme.min.css, but when I edit that file, nothing changes.

Thank you!

Figured it out: it needs to be changed in theme.css