Feature: Dark mode theme

A dark mode theme would be nice for the ones trying to lookup documentation in the middle of the night :)

The issue names no files, tests, or theme entry points. Start by locating the documentation app's frontend styling and theme configuration, then determine how dark mode should be selected and applied. Done means the documentation can be read comfortably in a dark theme, with the behavior verified in the app.

索引モデルが issue の本文から書いたものです。