[feat] Adding support for Dark Mode

[castastrophe]

By incorporating @media (prefers-color-scheme: dark) into our code and using that to invoke our dark mode settings, we can add support to all our components for the user preference for navigating in "Dark Mode". This has decent browser support and has received a lot of publicity. Would be a nice-to-have feature.

[castastrophe]

@coreyvickery I'd love to chat more about what you'd like the ideal "dark mode" experience to be for our patterns.

[coreyvickery]

@castastrophe That would be great. I know that @mceledonia has done a lot of work on dark theme on the PatternFly side, so he may have good ideas on how to roll it out.

[bennypowers]

I'd like to see this work using css vars / themes

we can also consider the approach shoelace takes

[coreyvickery]

@bennypowers I can help define the correct values.