Move existing dark-mode css to work with css media query

[ascholerChemeketa]

This creates duplicates of css rules targeting .pretext[data-atmosphere="dark"] and moves them to @media (prefers-color-scheme: dark)

Very hacky preview of what existing dark mode rules do.

[rbeezer]

Wild. Beta up at https://pretextbook.org/beta/html-dark-2024-01-26