add stylesheet for message students dialog to srgb

fixes CNVS-11066

test plan
=========
- navigate to screenreader gradebook and select an assignment
- click the "Message Students Who..." button
- verify that the dialog is styled the same as the one for the same
  assignment in GB2

Change-Id: I6006c31047f0fe3dcc6ab8289b983a2ca1713dd4
Reviewed-on: https://gerrit.instructure.com/30057
Reviewed-by: Matthew Irish <mirish@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>
QA-Review: Matthew Irish <mirish@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>