Used to be controlled by
<element id="TabsBar.TabCloseButton.Default" fg="xxx,xxx,xxx" /> and
<element id="TabsBar.TabCloseButton.Highlight" bg="xxx,xxx,xxx" fg="xxx,xxx,xxx" bdr="xxx,xxx,xxx" />
but these overrides no longer work in MB3.
fg from appropriate highlight tab
eg. <element id="TabsBar.Tab.Highlight" bg="xxx,xxx,xxx" fg="xxx,xxx,xxx" bdr="xxx,xxx,xxx" />
is used as 'hover-over' colour
normal (default) colour is auto-generated from it.