refactor(themes): add modular themes directory structure and docs

docs/themes/README.md

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+# Ionic Themes
+
+The Ionic Framework uses a modular theme system that separates core functionality from visual styling. Each theme is distributed in the [@ionic/theme](https://www.npmjs.com/package/@ionic/theme) package.
+
+## Directory Structure
+
+```bash
+core/
+└── src/
+    └── themes/
+        ├── base/
+        │   ├── default.tokens.ts   # Base design tokens shared across all themes
+        │   ├── dark.tokens.ts      # Dark mode tokens shared across all themes
+        │   └── light.tokens.ts     # Light mode tokens shared across all themes
+        │
+        ├── ionic/
+        │   ├── default.tokens.ts   # Ionic theme default design tokens
+        │   ├── dark.tokens.ts      # Ionic theme dark mode tokens
+        │   └── light.tokens.ts     # Ionic theme light mode tokens
+        │
+        ├── ios/
+        │   ├── default.tokens.ts   # iOS theme default design tokens
+        │   ├── dark.tokens.ts      # iOS theme dark mode tokens
+        │   └── light.tokens.ts     # iOS theme light mode tokens
+        │
+        ├── md/
+        │   ├── default.tokens.ts   # MD theme default tokens
+        │   ├── dark.tokens.ts      # MD theme dark mode tokens
+        │   └── light.tokens.ts     # MD theme light mode tokens
+        │
+        ├── functions.color.scss    # Sass color utility functions
+        ├── functions.font.scss     # Sass font and typography functions
+        ├── functions.sizes.scss    # Sass sizing and spacing functions
+        ├── functions.string.scss   # Sass string manipulation functions
+        │
+        └── mixins.scss             # Sass mixins
+```
+
+## Available Themes
+
+### iOS Theme (`@ionic/theme/ios`)
+- **Design System**: iOS Human Interface Guidelines
+- **Usage**: `import '@ionic/theme/ios'`
+
+### Material Design Theme (`@ionic/theme/md`)
+- **Design System**: Material Design
+- **Usage**: `import '@ionic/theme/md'`
+
+### Ionic Theme (`@ionic/theme/ionic`)
+- **Design System**: Ionic-branded design system
+- **Usage**: `import '@ionic/theme/ionic'`