I think that's bg of Panel.HeaderButtonOverride.Highlight
That's what I thought, but no, it doesn't work
These are my skin's lines for address bar:
<element id="Panel.Header.Default" bg="250,250,250" fg="0,0,0" bdr="250,250,250" />
<element id="Panel.HeaderControl.Default" bg="250,250,250" fg="0,0,0" bdr="250,250,250" />
<element id="Panel.LocationBarEdit.Default" bg="250,250,250" fg="0,0,0" />
<element id="Panel.HeaderControl.Highlight" bg="250,250,250" fg="250,250,250" bdr="250,250,250" />
<element id="Panel.HeaderButtonOverride.Default" fg="140,140,140" />
<element id="Panel.HeaderButtonOverride.Disabled" fg="200,200,200" />