Question

For some reason the background does not appear.

This is the CSS

div.sidebar_beige {
background: no-repeat left top;
height: 142px;
margin: 35px 0 35px 0;
position: relative;}

div.sidebar_beige h3 {
    color: #f57f20;
    font-family: 'trebuchet ms';
    font-size: 17px;
    left: 15px;
    margin: 0px;
    padding: 0px;
    position: absolute;
    top: 15px;
    width: 163px;
}
div.sidebar_beige p {
color: #454343;
font-family: 'trebuchet ms';
font-size: 12px;
left: 15px;
margin: 0px;
padding: 0px;
position: absolute;
top: 42px;
width: 165px;}

div.sidebar_fcs {
 background-image: url('../images/sidebar_backup_online.png');
}
div.sidebar_lap {
 background-image: url('../images/sidebar_backup_laptoprepara.png');
}

This is the html used:

<div class="sidebar_lap sidebar_beige">
            <h3>
              Laptop Reparatie
            </h3>
            <p>

              U kan niet gelijk een nieuwe<br />
            laptop kopen? <br />
            We zijn de goedkoopste<br />
            van Friesland.
            </p>
          </div>

Answer 1

You can use the order of overrides to your advantage here:

Make the copied.css just include:

div.sidebar_beige_lap {
  background: url('../images/sidebar_backup_laptoprepara.png') no-repeat left top;
}

In your page, include them in order:

<link rel="stylesheet" href="original.css" />
<link rel="stylesheet" href="copied.css" />

Any properties overridden in copied.css will win, otherwise properties from original.css are inherited.