Change Background

it really depends on how you want to style it but this will add background for content and sidebar area.

#content {
    padding: 20px 10px;
    border: 1px solid #ddd;
    background: #fff;
}

Add this code to Theme Options – Other – Custom CSS

Thank you. That was it! One more thing. How to draw the lines between posts?

What I meant was all the same as in sparking separate topic:
Posts in boxes, the side bar box too. just like in your theme Sparking.