improve contrast for account course text
improve contrast for account course text

test plan: text under courses on /accounts/X should meet
           contrast guidelines

fixes CNVS-23627

Change-Id: Ifa9af0a7532eac835f3f9102fbbf13019afba288
Reviewed-on: https://gerrit.instructure.com/64687
Tested-by: Jenkins
Reviewed-by: Dan Minkevitch <2591e5f46f28d303f9dc027d475a5c60d8dea17a@instructure.com>
QA-Review: Jahnavi Yetukuri <ed2b2672ccc2e59e56c0615df7f01b579202e014@instructure.com>
Product-Review: Cosme Salazar <d37f5e06cd46b8fd23ce5553334e09ede5394232@instructure.com>

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