Spinner design followup (PowerGreen only vs. changeable color)

[msmithNI]

😯 Problem to Solve

Followup from #822 for #346

The XD design for the spinner has a "Color UI Alt" version that makes the spinning bits the PowerGreen color. There's discussion/ an open question in #837 on how we should expose that variant:

• Have we shipped apps using that color variant already / do we already have specific apps in mind that will use that specific color for a spinner? (Brandon to answer this)
• Do we instead need a more general way for apps to provide an arbitrary/custom color for the spinner, to match their app theme/ background?
  • Dev note: In this case, we'd make a new token like spinner-color that can be overridden, and follow the same patterns we did for the icon colors which also allow overrides.

[NIbokeefe]

I'm still looking into where the power-green color scheme has been used.

I created an update to the design spec including additional sizes for the "bits-squares."

Sizes now include:

16, 32, 48, 64, 96, 128

[NIbokeefe]

We'll move forward without the Powergreen version of the bits spinner. I have not found where it's implemented.

[nate-ni]

Also, we have returned to the original sizes with 16, 32, 64.