adjust alert color contrast in classic quizzes
adjust alert color contrast in classic quizzes

fixes QO-870
flag=none

test plan:
 - view an alert in classic quizzes (one easy way is to
   preview a quiz)
 - confirm the light orange background color is #F9ECE8
 - confirm the dark orange text color is #C23C0D
 - these values should be unchanged for high constrast mode
   and should meet contrast requirements in both regular
   and high contrast mode.

Change-Id: Ia618e8fef6bd8f4313f868e00788cd4abcc7997e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/289637
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Product-Review: Marissa Pio Roda <f37c5aa2df660f6d74a722dca6aa783f0b56b74e@instructure.com>
Reviewed-by: Jorge Arteaga <797aca0b58c1608d2f0b1cb4d451b10108621a94@instructure.com>
QA-Review: Mark McDermott <20dd8ba615f2257ea7254f93d6f30ae11aa4f904@instructure.com>

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