Background Color

I talking about white parts behind images ….

You really need to be more specific…which page, what section?

Start with:

.main_container {
background: #000;
}

CSS background

When editing CSS, use a Child Theme, Custom CSS Plugin, or Theme provide custom CSS option – edits to parent themes are lost on theme update.

Learn CSS.

Do discover what CSS is output by your theme, use a web inspection tool such as Firebug, Chrome Inspect Element or IE9+ F12 Dev. Tools.