Publish dark mode variables.css

[bollwyvl]

Problem

I would like to embed widgets in light/dark aware applications, such as the pydata-sphinx-theme for ipywidgets on ReadTheDocs.

Proposed Solution

• Ship a variableslight.css and variablesdark.css
• re-enable the theme switcher in conf.py
• add some documentation about general integration strategies
  • as well as that specific theme

Additional context

• noted as follow-on work in Overhaul docs toolchain #3673