Dark theme from CSS media query

@josdejong What are your thoughts on providing the dark theme via `@media (prefers-color-scheme: dark)` instead of having to add the `jse-theme-dark` class?

The issue proposes providing the dark theme through the CSS prefers-color-scheme media query instead of requiring the jse-theme-dark class. No files, tests, or entry points are named; first locate the existing theme handling, then determine how automatic dark-mode support should coexist with the current class-based approach. Done should be clear from the resulting theme behavior.

Written by the indexing model from the issue text.