Refining Color Variables

CNVS-16082

This refines our color variables to make sure the same hues are
being used across flags.

Test Plan:
- Take a look at the styleguide and see the new contrast of colors,
along with better documentation on how to use color variables in
Canvas.

- I've touched a few specific areas for accessibility, consolidating
the color of neutral they're using. Essentially there is no change
for them, and we are bringing the change from HC into regular mode

Change-Id: Id33622fe7acc391854b673e994f36e6c9e2637d0
Reviewed-on: https://gerrit.instructure.com/43475
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>
QA-Review: Colleen Palmer <colleen@instructure.com>