[Lynne]

None of those are ".textcontrols a, a.textcontrol"

It's here in vbulletin.css:

.textcontrols a,
a.textcontrol {
    position:relative;
    padding: 3px {vb:math {vb:stylevar padding}/2};
    font: {vb:stylevar control_font};
    line-height: {vb:stylevar line_height};
    text-decoration:none;
    border: {vb:stylevar control_border};
    color: {vb:stylevar control_color};
    background:{vb:stylevar control_background};
    -moz-border-radius:{vb:stylevar popupmenu_border_radius};
    -webkit-border-radius:{vb:stylevar popupmenu_border_radius};
}

Now you can see the background is set by the stylevar "control_background". I'd suggest watching that video and also reading this which has the same info only written - How To: Find the correct StyleVar/LayoutVar to edit