Trying to edit CSS for gridmarket theme

I am trying to edit the Gridmarket theme.
I can't seem to locate where to make the changes I am trying to make.

I want to make the background image show starting at the top of the page rather than under the header.

The header has background colors, if I can eliminate them the header will show. But when I delete them from the CSS in the styles folder (color based styles) nothing changes.

This is confirmed NOT to be a cache issue. I am just not editing the correct file.

So... Any pointers on where to do this? Or is there a CSS editing tool that helps me find things like this?
I am OK with CSS, but have never edited WP theme CSS so I may be missing some basics.