No files under frontend/ (HTML/CSS/JS) were modified. Black is a Python formatter, so this feature only touches the backend and dev workflow.

• Added black as a dev dependency (uv add --dev black) and a [tool.black] section in pyproject.toml (line length 88, py313, excludes chroma_db).
• Added scripts/format.sh (auto-format backend/ and main.py) and scripts/check.sh (black --check plus the pytest suite).
• Reformatted all Python files with black (16 files; formatting only).
• All 61 tests pass after reformatting.

• ./scripts/format.sh: format the code
• ./scripts/check.sh: verify formatting and run the tests

Frontend formatting (e.g. Prettier for JS/CSS) was not added, since it wasn't requested.

• Added a #themeToggle button (fixed, top-right) with inline sun and moon SVG icons. It has aria-label and aria-pressed, and is a native <button>, so Tab/Enter/Space work.
• Added a small inline script in <head> that applies the saved theme (localStorage) to <html data-theme> before first paint, which avoids a flash of the wrong theme.
• Bumped the CSS and JS cache-busting versions.

• Added a :root[data-theme="light"] block that overrides the CSS variables: light backgrounds and surfaces, dark text, a darker teal primary (white text on it passes WCAG AA), and adjusted borders and focus ring.
• Replaced hardcoded colors with new variables so both themes work: --on-primary, --on-user-message, --link-hover, --link-underline, --user-link-underline, --code-bg, --welcome-shadow, --error-*, --success-*, --button-glow. Dark values equal the previous hardcoded ones, so the dark theme looks unchanged.
• Fixed the blockquote border, which referenced the undefined --primary.
• Added .theme-toggle styles: a circular button with hover and :focus-visible ring. The sun and moon icons cross-fade and rotate.
• Added an html.theme-transition rule that animates background, text and border colors for 0.35s.

• Added setupThemeToggle(), called on load. A click flips data-theme on <html> between light and dark, saves it to localStorage, updates the button's aria attributes, and applies the temporary theme-transition class so the change is smooth. The class is removed afterwards so normal interactions aren't slowed.

• Toggled by click and by keyboard (Enter), checked persistence across reloads, and checked desktop and 390px mobile layouts in both themes.
• Measured WCAG contrast for 20+ text/background pairs in both themes (messages, links, code, blockquote, sources, error/success, sidebar, buttons, input). All are at least 4.5:1 after these fixes:
  • body background had been accidentally set to --on-primary and is now --background.
  • Links in user messages now use --on-user-message (was 3.18:1 in light).
  • Light --success-text darkened to #166534 (was 4.2:1).