Increase due date color contrast in Syllabus table

Fixes: CNVS-14603

Test plan:
- Add some assignments to your course, if you do not
  have them already
- The background colors of the table are created with
  a combination of css and background images, so I have
  created http://cl.ly/image/1D1V2m1h2B3v/CNVS-14603.png
  to show you the colors to test against using the
  WebAIM contrast checker:
  http://webaim.org/resources/contrastchecker/
- The text colors should now be #666 (default UI) and
  #333 (high contrast)
- For default UI, the required contrast is at 3:1; for
  high-contrast UI, the requirement is 4.5:1

Change-Id: I880c8144f4dd2c1b4b91349b49f073c9f1bc9f10
Reviewed-on: https://gerrit.instructure.com/39555
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>