adjust warning alert colors

fixes CNVS-10844

Change-Id: Ia8b6d1966127d2ded89f0864914ea5c9d85bce8c
Reviewed-on: https://gerrit.instructure.com/30707
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>