Make a plugin for this:
AdminCP > Plugins & Products > Add New Plugin
Product: vBulletin
Hook location: parse_templates
Title: Custom Style Changes
Execution Order: 5
Code:
if (STYLEID == 1)
{
$template_hook['headinclude_bottom_css'] .= '<link type="text/css" rel="stylesheet" href="aaa.css' . '?d=' . $style['dateline'] . '" />';
}
1 is the style you are trying to do this to,
aaa.css is the name of your stylesheet.