Bug - Dark/light theme toggle button contrast

[srambach]

Describe the problem
The dark/light theme toggle in the top toolbar is using black icon color on the blue selected toggle color. The toggle component works fine, so this is likely a leftover workspace style that is breaking it.

https://staging-v6.patternfly.org/components/toggle-group

@jknilans

Screenshots

[evwilkin]

Closed by #4203