Change background color on wp-content/uploads/myimage.jpg

I think this may be a generic wp question but I'm using Fixer theme so...

I'm come across an issue with Upfront, I need the background of my site to be ALLWASY black, I set the global bg in Upfront so everything looks fine...

Unfortunately I need to have some images linked to the media uploads because responsiveness in galleries and lightboxes in that theme are just not working at all :slight_frown:

How can I make the background of a linked image also black?

by default is white and changing body css background does not work