This is a 2 parter.

Part 1:

Go to /templates/crystal/css/units.css ( depending on which template you use)

On line 71 find:

background-position: top left;

    width: 9px;
}

.rounded_mid_center {
    padding: 0px;
    background-color: #f8f8f8;
}

change:
background-color: #f8f8f8 to transparent

Part 2:

Go to /templates/crystal/css/wrappers.css

Find this section:

background-position: top right;
    background-repeat: repeat-y;

}

.bg_mid_center {
    background-image: url(../images/bg_mid_center.png);


change the image name to your own image.

Mine is:

background-position: top right;
    background-repeat: repeat-y;

}

.bg_mid_center {
    background-image: url(../images/background-image: url(../images/SpiderWebRed1.gif);

Change this image; bg_mid_center.png

Now refresh your page and you will see your background in the blocks.