MatStepper: Step header focus treatment is not strong enough

[glushkova91]

Reproduction

Open any example with Stepper material component:
https://material.angular.io/components/stepper/overview
Focus on it and check background color for active state (rgba(0,0,0, .04))
For white background it's #f5f5f5
https://webaim.org/resources/contrastchecker/?fcolor=F5F5F5&bcolor=FFFFFF

Expected Behavior

Color contrast between background for focus state and page background will be at least 3:1

Actual Behavior

Color contrast is 1.09:1

[jelbourn]

To clarify here: the text contrast ratio is fine, but the actual focus treatment is not sufficiently strong enough to signal focus state.