feat(themes): new color theme engine & packs 🎨

- **New Hybrid Engine:** Fully data-driven theming using YAML (`modules/blox-tailwind/data/themes/`). Supports:
  - **Palette Mode:** `primary: "indigo"` loads exact Tailwind 50-950 scales (OKLCH) for perfect utility matching.
  - **Hex Mode:** `primary: "#brand"` auto-generates accessible 50-950 tint/shade scales.
  - **Semantic Overrides:** `ui.header.background` accepts gradients, transparency, and images.
  - **Neutral Mapping:** Themes can retarget the gray scale (e.g. `neutral: "stone"` warms up the entire UI).

- **New Theme Packs:**
  - `default` (Blue/Sky + Dark)
  - `contrast` (High Contrast Light)
  - `latte` (Warm Coffee/Stone)
  - `dracula` (Dark Purple/Pink)
  - `americano` (Dark Espresso)
  - `marine` (Deep Ocean/Teal)
  - `retro` (Vintage Paper)
  - `synthwave` (Neon/Purple)
  - `cupcake` (Light Pink/Fuchsia)

- **Architecture:**
  - Standardized on `appearance.theme.light` / `appearance.theme.dark` config.
  - Legacy `appearance.color` remains as a primary color override.

This overhaul enables colored backgrounds/navbars, enables mix-and-match themes, and future-proofs the styling system for Tailwind v4.

templates/documentation/config/_default/params.yaml

@@ -6,7 +6,9 @@
 # Appearance
 appearance:
   mode: light
-  color: blue
+  theme:
+    light: "default"
+    dark: "default"
 
 # SEO
 marketing: