Accessibility color on quiz stats

This updates the colors used on the quiz statistics so they
can pass accessibility needs.

fixes CNVS-13982

Test Plan:

- Recompile css - npm run compile-sass
- Make sure New Quiz Statistics feature is active
- As an admin or teacher that has given a quiz to students, go to
the "Statistics" tab to view new statistics
- Using inspector, grab the hex for the Gray color used on the
"Avg Score" title
- Go to Webaim and enter that color as foreground and #ffffff (white)
as background and make sure it passes WCAG AA

Change-Id: Id53d68991dba2fecee3792d8f14f5cc4841a72b3
Reviewed-on: https://gerrit.instructure.com/39223
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>