[rich-iannone]

This PR adds light/dark theme pairs to termshow recordings: theme: auto uses the built-in github-light/github-dark pair, and theme: {light, dark} lets authors choose their own pair, with each side a theme name or an inline mapping. The manifest stays at version 2 and gains an optional dark member that holds the dark theme's colors and rows in parallel to lines, so a pair adds a second set of colors rather than a second set of frames (SVG files and posters use the light side).

The player draws the side that matches the page's color mode (data-bs-theme, the Quarto classes, or prefers-color-scheme) and redraws when the mode changes. Themes also gain an optional cursor color, and the player's light-mode CSS, whose body[data-bs-theme] selectors never matched on Great Docs sites, now works through a class the player sets itself.

The schemas, the Terminal Recordings guide (with a live theme: auto demo), the format spec, and the tests (including node parity tests for dark-side drawing and mode detection) are now updated to match.

[github-actions]

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