fix success flash color contrast

closes CNVS-6675

previous green color didnt have enough contrast for WebAIM - WCAG

Change-Id: I36ae6eaddff0d622eb62e043709753893bed55e7
Reviewed-on: https://gerrit.instructure.com/32978
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>