## Why

Add modern theming controls (dark/light/high-contrast) to improve accessibility and give the site a polished, customizable "WOW" experience.

## What Changes

- Add three user-selectable themes: **Dark**, **Light**, and **High Contrast**.
- Add a floating theme switcher "notch" on the right edge of the screen:
  - positioned just below the primary nav bar
  - leaves enough vertical space for secondary navigation
  - hover state includes a tasteful animation
- Theme switching uses a smooth transition (not an abrupt flash).

## Capabilities

### New Capabilities
- `site-theming`: Theme tokens and a theme application mechanism that can switch between Dark/Light/High Contrast across the site.
- `theme-switcher-notch`: A floating, accessible UI control (right-side notch) that lets the user switch themes.

### Modified Capabilities
- `wcag-responsive-ui`: Extend accessibility baseline to include theme switching requirements (keyboard, focus, reduced motion) and ensure High Contrast theme is supported.

## Impact

- Affected UI/CSS: global design tokens (CSS variables), background layers, card/CTA styling, focus styling.
- Affected layout: a new floating notch component that must not overlap navigation across breakpoints.
- Affected UX/accessibility: keyboard navigation and motion preferences during theme transitions.
