Change primary nav modal overlay color
Change primary nav modal overlay color

Fixes: CNVS-24302

Test plan:

- This is new UI only
- Go into Theme Editor and make your primary
  navigation a brand-new color. Save your
  changes.
- Leave Theme Editor and click the Courses link
  from the primary navigation
- No matter what you chose for a nav color, the
  overlay when the navigation drawer is open
  should be black at 50% opacity:
  rgba(0,0,0,0.5)
  http://screencast.com/t/Vw4G6dIO2xcX

Change-Id: I29e9a22ab1ec12e8dd7de4618d9dd0da41ad4fd2
Reviewed-on: https://gerrit.instructure.com/67511
Tested-by: Jenkins
Reviewed-by: Jennifer Stern <7e594e5cba47371ac9ee694a84b9d94f9f6ec815@instructure.com>
Product-Review: Jennifer Stern <7e594e5cba47371ac9ee694a84b9d94f9f6ec815@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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