main-rollup.css: Where are these definitions?

Aha! It's actually the tabslight_border Stylevar. It was being all sneaky, and hiding in the CSS with a stylevar call in the defintion. I just wasn't going about it with the right thought process (I was hunting for absolute code, HTML colors, RGB colors, needles, haystacks, and the remainder of my sanity...)

.formcontrols .blockrow {
	background:{vb:stylevar formrow_background.backgroundColor} {vb:stylevar formrow_background.backgroundImage} bottom repeat-x;
	border-top:solid 1px {vb:stylevar light_border.borderColor};
	padding:10px 5px;
}

I'm just going to change my signature to "Thank you Lynne, I owe you a(nother) beer" at this point.