created new styling for CNVS-14751

new colors are based on $canvas-neutral

contrast for context on assignments tab. can be viewed at 'courses/1/grades/3#tab-assigments'

new default: #555555 on #CDF - 5.45:1
new high-contrast: #070707 on #CDF - 14.73:1

Change-Id: I2cb195a5979f18b78de3d528c2623463d912f4d9
Reviewed-on: https://gerrit.instructure.com/41001
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Chris Hart <chart@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>