menu-bar: does not support dark mode themes

[Splaktar]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Dark mode themes work on all components.

What is the current behavior?

Dark mode themes work on most components, but not menu-bar. This is related to the same issue (#11199) affecting menus.

What is the use-case or motivation for changing an existing behavior?

Consistent support for dark mode across components.

Which versions of AngularJS, Material, OS, and browsers are affected?

AngularJS 1.6.9
AngularJS Material 1.1.8
All browsers

Is there anything else we should know? Stack Traces, Screenshots, etc.

I can't find any dark mode examples on https://material.io/guidelines/components/menus.html#menus-specs, but I think that https://material.angular.io/components/menu/overview#nested-menu can be used as a guide.

Currently with the fixes from #11230:

Angular Material:

[Splaktar]

Here's another more complete screenshot in dark mode since md-menu-bar also affects the md-toolbar theming: