how to make categories sidebox of my own style?

[gjh42]

There is (or should be) already a <hr> displayed at the beginning of the all/featured/new section:

  if (SHOW_CATEGORIES_BOX_SPECIALS == 'true' or SHOW_CATEGORIES_BOX_PRODUCTS_NEW == 'true' or SHOW_CATEGORIES_BOX_FEATURED_PRODUCTS == 'true' or SHOW_CATEGORIES_BOX_PRODUCTS_ALL == 'true') {
// display a separator between categories and links
    if (SHOW_CATEGORIES_SEPARATOR_LINK == '1') {
      $content .= '<hr id="catBoxDivider" />' . "\n";
    } 

I think I remember seeing the switch for this in admin, but can't locate it now, and a search on SHOW_CATEGORIES_SEPARATOR_LINK finds only this mention in the code; no define or switch.