created new styling for CNVS-14751
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Chris Hart <b82b40b84a2f06957ec9caf1768340bd6b929cc4@instructure.com>
QA-Review: Nathan Rogowski <2e8aa918660411855c6d44d5bb2da677aa033255@instructure.com>
Product-Review: Colleen Palmer <387ba1943871e60965e6b44638a2f2e1ab9b8469@instructure.com>

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