remove darker background from course homepage sidebar

fixes CNVS-12084

If you go to a course homepage, the right sidebar should only be one
shade of gray instead of it's previous two shades of gray.

Test Plan
1. go to a courses homepage.
2. make sure the background on the right sidebar is only one shade of
gray instead of two.
3. have a little party

Change-Id: If402c2bbe4b4d5301907d52ce6d56cbfd953a6fe
Reviewed-on: https://gerrit.instructure.com/32615
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>