update rubric table title background/border color with variables
update rubric table title background/border color with variables

Fixes: CNVS-37791

test plan:
- as a teacher/admin navigate to an assignment
- click the "+ Rubric" button to reveal the Rubric Table
- go thru the steps to enter a rubric name, etc...
  click "Create Rubric"
- the saved or created rubric table should now have a
  lighter gray background to accommodate the standard
  icon color (edit, find, delete)
- new view: https://screencast.com/t/s2Si3RVS

Change-Id: I91150feea9f703c2cb479eab8a8709d1f5e742a6
Reviewed-on: https://gerrit.instructure.com/117171
Tested-by: Jenkins
Reviewed-by: Chris Hart <b82b40b84a2f06957ec9caf1768340bd6b929cc4@instructure.com>
QA-Review: Dan Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Kyle Follett <3f0dcef4ea7407b413e459a87705152d37b86e63@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms