Changing background color of Brine gallery block caption

Working on a site and the client wants the captions in the gallery block I set up to have an opaque background, or just appear entirely underneath the photo. Anyone know how to create a solid background behind captions in a gallery block with css? Preferably that spans the entire image horizontally? I've got the caption position set to "bottom left" right now, which creates a transparent background, but it wraps the text and keeps everything on the left side. If I select "left" as the caption position, there's no background at all. 

So, I need css to either:

a) Make the background that appears in the "bottom left" caption position setting transparent AND make it span the full width of the image or
b) Create a background for the "left" caption position setting OR
c) Somehow detach the caption to appear underneath the image

Any ideas?

Can you share link to your site?

It is still in trial, so password protected