Slider thumb inner background should not be transparent

[charlessuh]

Description

For sliders with a ramp or tick, the circle thumb shows the ramp or tick behind it.

Steps to reproduce

spectrum-css:

Expected behavior

Design spec:

Screenshots

See above.

Environment

• Spectrum CSS version: @spectrum-css/[email protected]
• Browser(s) and OS(s): Chrome 88

Additional context

n/a