a11y(buttons): Low contrast on focused buttons

[crisbeto]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

The buttons should have sufficient color contrast, both in regular mode and in high contrast mode.

What is the current behavior?

Currently, the button focus indication is a tint with 12% opacity, which is barely visible in regular mode and is completely invisible in high contrast mode. This makes it difficult to see whether a button has been focused.

CC @jelbourn

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