That sidebox header is made from two images:

.category_sidebox_header_left{
background: url("../images/left_c_sb_bg.gif") no-repeat top left; padding: 0 0 0 4px;
height: 23px;
}

and



h3.category_sidebox_header_right{
background: url("../images/right_c_sb_bg.gif") no-repeat top right;}

You'll have to add those to the other sidebox headers (sidebox_header_left and right) the same way in the stylesheet.