fix(drawer): better handling of high contrast mode

[crisbeto]

Fixes the open drawer blending in with the app background in high contrast mode.

[andrewseguin]

The test failures on this look suspicious - can you look at them?

[crisbeto]

I doubt it was from these changes @andrewseguin since they'll only apply if the high contrast media query is matched. I've rebased it just in case.

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