Yeah you can, but you'll have to add it as a custom CSS rule in your own CSS file. This is quite easy to do as I'll now explain...

In a text editor create a new file and put in the contents:

#content, .title-divider, .title-divider span {
  background-color: #eee;
}

Save it as "custom.css" or something.

Then go to your site's admin panel, and go to Content Management -> Assets Library, and click on Add Custom Theme.

On that page click the Browse button and select your custom.css file. Call it whatever you like - "custom" will do.

Now that the CSS file is added, you then just need to configure the Modern theme to use it. So in the site admin pages go to Look & Feel, and click the Options button for the Modern theme.

On the Extra CSS File entry click Select from Assets Library, and select your "custom" file that you added earlier.

Then save the settings, and all being well the site will have a grey background for the main body.

I have done these exact steps on the Voice Demo Site and so you should see on there what effect it has. I think it looks good!

Thanks

Joe

I notice that on your site, some of the areas now don't look so good as they have white background instead of the grey you set up. So you could try this rule to get a bit more!

#PageBody, .title-divider, .title-divider span {
  background-color: #eee;
}

Joe