Bug Report

• Package version(s): (2.20.2)
• Browser and OS versions: (IE/Edge Win10)
  Microsoft Edge 40.15063.0.0
  Microsoft EdgeHTML 15.15063

Priorities and help requested (not applicable if asking question):

Are you willing to submit a PR to fix? (Yes, No)
No

Requested priority: (Blocking, High, Normal, Low)
High

Products/sites affected: (if applicable)
VSTS or any site using slider ( and users using it high contrast modes )

Describe the issue:

Slider: https://dev.office.com/fabric#/components/slider

Slider in non-high contrast mode

In "High contrast Black" and "High contrast White" [ could be issue in other high contrast modes as well] Slider is not visually similar to what we see in non-high contrast mode, Only the silder knob(circle) is visible in this mode .

Actual behavior:

High Contrast Black
Slider: https://dev.office.com/fabric#/components/slider

High Contrast White

Expected behavior:

In high contrast mode we should get the slider visually similar to what we get in non-high contrast mode.

User Impact:

User with low vision who relay on screen reader will face difficulty to access the slider.

If applicable, please provide a codepen repro: