Bugzilla

Changes for this bug update both the regular and first-time toggles to the new UI styles specified in the Figma link: https://www.figma.com/file/Pk2jgzwiVvaq3Gf5ERKuAL/Picture-in-Picture?node-id=2420%3A24145&t=zMii4OptzUNVP3AO-0. It seemed easier to cover both toggles at once.

The summary of changes includes:
  - New styling
  	- Updated styling for toggle shape and opacity
  	- Update styling for dark toggle; default theme is dark
  	- Added new styling for light toggle
  - New toggle behaviour concerning styling
  	- By default, toggle theme will follow `prefers-color-scheme`. So if a light scheme is found, light toggle will be visible
  	- Set up Nimbus for setting the PiP toggle theme; it will override color scheme preference
  - New tests
  	- One test for verifying Nimbus PiP toggle settings
  	- One test for updating toggle according to `prefers-color-scheme`

Some items still need confirmation:
  - What will happen to the “pip-explainer” for the expanded toggle
  - Final decision on how the PiP background will update (color-scheme, Nimbus, a new Firefox pref, etc.)

Open to feedback for the meantime. See this document for a sample Nimbus recipe and example manual testing workflows: https://docs.google.com/document/d/1zgzzDsIqv6VSAm9VjGTyn1cPwIELczxDTBX_89LFsIg/edit#heading=h.1mgf0sy3isby

Depends on D168847

Edited: the commit message I set for the second patch is the same as above.