Color contrast - Student Grades view

Fixes: CNVS-15567

Multiple small UI tweaks to increase foreground-
background contrast in the STUDENT view of the
Grades section in high-contrast mode.

IMPORTANT: This ticket is only for the high-
contrast UI. To enable the high-contrast UI, go
to Settings then scroll down and make sure
"Use High Contrast Styles" is set to ON.

Test plan:
- Enable high contrast UI in Canvas (see above)
- Go through each issue Nathan entered into the
  JIRA and compare with the updated UI to make
  sure it's fixed.
- The webAIM color contrast tester can be found
  here: http://webaim.org/resources/contrastchecker/

Change-Id: I7eea468504fb0f8be151bfb3417ad27fe0a0e4e6
Reviewed-on: https://gerrit.instructure.com/41828
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>