improve contrast for quiz moderate modal text
improve contrast for quiz moderate modal text

fixes QO-659
flag=none

test plan:
 - create a classic quiz with a time limit
 - click "Moderate this quiz"
 - click the edit icon for a student to open the extensions modal
 - all text in the modal should meet contrast requirements
 - enabling high contrast mode should make the text even darker

Change-Id: I471ac301b85996ff9713b03729192615fa230f6a
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/253996
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Bryan Petty <33564f6eff3d808d3eaaa056a1631fdcde5faed6@instructure.com>
QA-Review: Mark McDermott <20dd8ba615f2257ea7254f93d6f30ae11aa4f904@instructure.com>
Product-Review: Susan Sorensen <0d98e282ba669db948aa8e61bd608c56b3926d27@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms