Instead of creating two CSS provider objects, just import
the recipes.css from the cuisine.css, since we are rewriting
cuisine.css at runtime anyway.

At the same time, introduce separate recipes-light.css and
recipes-dark.css files, so we can do color tweaks for the
dark theme in the future.

And reload the CSS when the gtk-application-prefer-dark-theme
setting changes. This is not really a common occurrence, but
it is easy to do and it helps for debugging the CSS with the
GTK+ inspector.