Cranberry palette refresh + dark mode

[EmilHvitfeldt]

Summary

Adopts the Posit creative team's accessible cranberry color family (base #BA355F) and adds a light/dark theme toggle to the site.

Two logical pieces, sequenced so the palette refresh could stand alone:

1. Palette refresh (light) — swap the base cranberry #CA225E → #BA355F and key the light theme off a named seven-step ramp.
2. Dark mode — add a dark: theme with a navbar toggle, deriving the dark surface from the existing tidymodels ink (#1A162D), and make the previously light-only styles theme-aware.

Palette and contrast guidance come from Tidymodels-ColorGuidance-Accessibility-2025 (Posit creative team), documented against WCAG 2.2 AA.

What changed

• _cranberry.scss (new): shared color ramp as SCSS variables ($cranberry-light-1 … $cranberry-dark-3, surfaces, inks, $theme-blue/$theme-blue-dark), layered into both themes.
• styles.scss: base cranberry #BA355F, link/hover/focus-ring and table tints keyed off the ramp.
• _quarto.yml: theme: moved under format: html: as a light:/dark: mapping; respect-user-color-scheme: true.
• styles-dark.scss (new): dark Bootstrap defaults + .quarto-dark-scoped rule overrides (h2, code/tables/session-info, resource stickies, and the front page bands/event card; decorative tidy-back-*.svg backgrounds hidden on dark).
• find/listing-cards.css, start/styles.css: .quarto-dark override blocks (plain CSS, ramp values inlined); start/styles.css mirrored value updated to #BA355F.
• learn/models/parsnip-predictions/index.qmd: page-level theme: converted to the mapping shape so it merges with the site light/dark themes.
• README.md: new "Color palette" and "Light and dark themes" styling docs.

Accessibility

Contrast ratios verified against the guidance (all pass their WCAG tiers):

role	color / surface	ratio
link (light)	#BA355F on white	5.58 (AA)
link (dark)	#CB6585 on #1A162D	4.80 (AA)
code (dark)	#919BCD on #1A162D	6.50 (AA)
body (dark)	#DDDDDD on #1A162D	12.92 (AAA)
focus ring	#BA355F on white	5.58 (>3.0 UI)

Notes / follow-ups

• Sass layering: _cranberry.scss is listed last (Quarto emits scss:defaults from later-listed files first); styles-dark.scss is listed before styles.scss so its variable overrides win, with rule overrides scoped under .quarto-dark for specificity. Documented in the README.
• Re-render pending: the parsnip-predictions freeze is invalidated by the front-matter fix and will re-render in CI. That first data-heavy render is the moment to review dark-mode output tables.
• Not yet done: live in-browser eyeball of the toggle across both themes.
• The decorative front-page illustrations still carry the old #CA225E and are simply hidden on dark; purpose-built dark variants are a possible future improvement.