You could modify column_left.php to wrap one or more divs around the first three sideboxes (or whatever grouping you want), and apply the parchment styling to those.
But the easier (and probably just as effective) approach would be to cut the parchment image into top, middle and bottom sections (with extra height for any box that could be variable in size) and apply them to the sidebox ids.
Code:
#search {
background: url(../images/parch_top.gif) no-repeat;
margin-bottom: 0;
border: none;
}
#categories {
background: url(../images/parch_middle.gif) repeat-y;
margin-top: 0;
margin-bottom: 0;
border: none;
}
#information {
background: url(../images/parch_bottom.gif) no-repeat;
margin-top: 0;
border: none;
}
Setting the adjoining margins to 0 will make the boxes appear as one seamless unit. You would also want to set their headings to have no background/borders etc.