use standard font color in discussions.
use standard font color in discussions.

fixes CNVS-4242

test plan:
  * create a discussion;
  * post to the discussion;
  * verify that the post text is in the standard canvas
    dark grey color (#333);

Change-Id: I06a3f874ba1a3358a00eacf09dbfc49128c69031
Reviewed-on: https://gerrit.instructure.com/18150
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jon Willesen <eb5ce5e6a76c243326fecfa94534101600f87bd9@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>

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