ds-healthcare-gov Header menu does not render background-color in certain configurations

[bernardwang]

The ds-healthcare-gov site package provides a Header component, and that
Header does not render a background-color for the dropdown menu in
certain configurations. It looks like background-color is only being
set when a subhead value or loggedIn=true is applied. This means you
cannot have a basic header with only a list of links, e.g.
<Header links={sitewideLinks} />, or you will have white text with no
default background.

See also:
https://github.cms.gov/CMS-WDS/ds-healthcare-gov/blob/master/src/components/Header/_Menu.scss#L81

To reproduce, you can visit
https://github.cms.gov/pages/CMS-WDS/ds-healthcare-gov/patterns/header/
and change the inline example to the "XS" breakpoint and select the
"Home" variation. Then click the "Menu" button. You won't see any
visible change, but "TODO: defaultMenuLink.js" is displayed in white
text on a white background. You can click "Inverse Header" to change
the body bg color to dark blue, and you will see the default menu link.

Tracked via https://jira.cms.gov/browse/WNMGLEARN-398.