Add listening for changes to all theme features in preferences controller

See https://github.com/flutter/devtools/pull/8336

Preferences controller currently manages dark/light mode changes, but should also manage background color, foreground color, and font size to allow listeners to update when those theme features change.

Start with the preferences controller and review the related changes in pull request 8336. Confirm how dark/light mode changes are currently observed, then ensure background color, foreground color, and font size changes also notify listeners; done means all listed theme features trigger updates.

Written by the indexing model from the issue text.