Increase text contrast in right sidebar
Fixes: CNVS-14602

Test plan:
- Trigger a to-do in the right sidebar
- Ensure that the color contrast meets WebAIM
  standards of 3:1 in the default UI and at least
  4.5:1 in the high-contrast setting.
- Note: the body background image is around #e5e5e5,
  which is what I used to test the ratios while
  working on this.

Change-Id: Ic4188d7d633da177abadbb5b3320ce0f70b9ac99
Reviewed-on: https://gerrit.instructure.com/39155
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>