Removing padding around .inside-article

I'm in the process of creating a Child Theme with a full-width Page Template.
So the template I'm creating is specifically built to have no gap between the horizontal edges of the content and the viewpane itself.

In GP, content is contained within the div with the class '.inside-article'.

It's easy enough for me to create a rule in my own css that looks like this:

.separate-containers .inside-article {
padding: 0 !important ;
}

However, !important should be a tool of last resort.
Besides, I can see that the CSS that I'm overriding is inlined into the page itself.

This makes me think that the inlined CSS is generated from somewhere in the GP settings. So overriding with !important will cause me pain further down the road.

However, I can't find a setting that removes or allows me to set the padding.

Where is it? How do I find it?