Darken webkit scrollbar color for high contrast UI
Darken webkit scrollbar color for high contrast UI

Fixes: CNVS-27352

We only control the scrollbar color for webkit
browsers (Safari/Chrome), so I have darkened the
custom color significantly for the high contrast
UI: http://screencast.com/t/KpgZ82DC8jpg

Test plan:
- Open Safari or Chrome with Canvas running
- Turn on the high contrast UI for your user
- Find a page with scrollbars. The background
  color of the scrollbar handle should be set
  to our $ic-color-dark variable (#333 right now).

Change-Id: I35825059055955ed9151758594b6062d94e137fb
Reviewed-on: https://gerrit.instructure.com/72530
Tested-by: Jenkins
Reviewed-by: Pam Hiett <7b533437add1ce4ea4ed79979fa88e0e484e554b@instructure.com>
Product-Review: Pam Hiett <7b533437add1ce4ea4ed79979fa88e0e484e554b@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>

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