Color of Background versus color of Body (or div???)

I'm not a coder.

Here’s something that’s been on my wish list for a while… If you look at the screenshot below, I want to keep the background of the body white.  (By "body", I mean the part of the canvas that has content.  (In default.css, I've set the max-width to 775px.)

But I want the background of the UNUSED part of the page to be gray (so that the page/content itself is more defined, kind of like when you view a PDF via a browser -- the page of the document itself is white, but the BACKGROUND on both sides of the page is dark gray.)

I'm not a coder.  Seems it would be via CSS code rather than HTML in Master Pages...?

I've emailed this question to a developer who does not use RoboHelp.  Her response was:

However, I haven't been able to implement this, because 1)  I cannot figure out exactly where, and in which file, to put the line about linear-gradient; and 2)  I cannot find a div.top section default.css, layout.css, or MasterThemeSchema files.

Any ideas?

Thanks much,

Mark Thiel