This mod uses the primitive structure of the original Zen Cart categories sidebox, which is not built to allow an entire line to be highlighted.
That said, it is possible to alter the display with the help of some admin settings and some stylesheet rules to do this. It would not be practical with a more complex category structure than you have.
Code:
/* CATEGORY SECTION NEED TO FIGURE THIS OUT */
#categorylistbox {
background: #c00000; border: 1px dashed #aabb00; border-radius: 1.5em;
}
#categorylistboxHeading {
background: #00f0f0; border-radius: 22px 25px 0 0;
}
#categorylistboxContent {
background: #c0f0f0; border-radius: 0 0 1em 2em; padding: 0;
}
#categorylistboxContent span {
display: block; height: 1.4em;
}
#categorylistboxContent a {
display: block; height: 1.3em; padding-left: 0.3em;
}
#categorylistboxContent .category-products {
padding-left: 1.4em;
position: relative; top: -1.4em;/*this is to counter a puzzling vertical offset that ought not to be happening... due to the span? */
}
#categorylistboxContent a:hover {
background: #70f0f0; display: block;height: 1.0em;
}
#categorylistboxContent span+br {
display: none;
}
Again, adjust to taste. Don't just leave everything as I post it, as some of these are exaggerated to show the effect.