Support changing foreground color and opacity for focusingItem, visibleRange, and inactiveRange

[joeyfigaro]

Is your feature request related to a problem? Please describe.
Setting the background color for the entire visibleRange container makes for poor scannability.

Describe the solution you'd like
I'd like to be able to control the background, opacity, and text/foreground color of the visibleRange, and focusingItem. I'd also like support for lowering the opacity of items that are not in the visibleRange to raise the contrast of visibleRange items.

Describe alternatives you've considered
Looked at custom css, but it's unclear what selectors we should be using–if any. Color settings default to CSS vars, so unsure if that's all we're expected to use.

Additional context