Add built-in theme support with various pre-defined themes

- Introduce multiple built-in themes (`default_dark`, `default_light`, `gruvbox`, `dracula`, `solarized`, `midnight-ocean`, `rose-pine`, `monokai`, `material-dark`, `material-light`).
- Implement theming system with customizable color schemes for all UI components in the TUI.
- Include documentation for themes in `themes/README.md`.
- Add fallback mechanisms for default themes in case of parsing errors.
- Support custom themes with overrides via configuration.

themes/README.md

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+# OWLEN Built-in Themes
+
+This directory contains the built-in themes that are embedded into the OWLEN binary.
+
+## Available Themes
+
+- **default_dark** - High-contrast dark theme (default)
+- **default_light** - Clean light theme
+- **gruvbox** - Popular retro color scheme with warm tones
+- **dracula** - Dark theme with vibrant purple and cyan colors
+- **solarized** - Precision colors for optimal readability
+- **midnight-ocean** - Deep blue oceanic theme
+- **rose-pine** - Soho vibes with muted pastels
+- **monokai** - Classic code editor theme
+- **material-dark** - Google's Material Design dark variant
+- **material-light** - Google's Material Design light variant
+
+## Theme File Format
+
+Each theme is defined in TOML format with the following structure:
+
+```toml
+name = "theme-name"
+
+# Text colors
+text = "#ffffff"              # Main text color
+placeholder = "#808080"       # Placeholder/muted text
+
+# Background colors
+background = "#000000"        # Main background
+command_bar_background = "#111111"
+status_background = "#111111"
+
+# Border colors
+focused_panel_border = "#ff00ff"    # Active panel border
+unfocused_panel_border = "#800080"  # Inactive panel border
+
+# Message role colors
+user_message_role = "#00ffff"       # User messages
+assistant_message_role = "#ffff00"  # Assistant messages
+thinking_panel_title = "#ff00ff"    # Thinking panel title
+
+# Mode indicator colors (status bar)
+mode_normal = "#00ffff"
+mode_editing = "#00ff00"
+mode_model_selection = "#ffff00"
+mode_provider_selection = "#00ffff"
+mode_help = "#ff00ff"
+mode_visual = "#ff0080"
+mode_command = "#ffff00"
+
+# Selection and cursor
+selection_bg = "#0000ff"      # Selection background
+selection_fg = "#ffffff"      # Selection foreground
+cursor = "#ff0080"            # Cursor color
+
+# Status colors
+error = "#ff0000"             # Error messages
+info = "#00ff00"              # Info/success messages
+```
+
+## Color Format
+
+Colors can be specified in two formats:
+
+1. **Hex RGB**: `#rrggbb` (e.g., `#ff0000` for red, `#ff8800` for orange)
+2. **Named colors** (case-insensitive):
+   - **Basic**: `black`, `red`, `green`, `yellow`, `blue`, `magenta`, `cyan`, `white`
+   - **Gray variants**: `gray`, `grey`, `darkgray`, `darkgrey`
+   - **Light variants**: `lightred`, `lightgreen`, `lightyellow`, `lightblue`, `lightmagenta`, `lightcyan`
+
+**Note**: For colors not in the named list (like orange, purple, brown), use hex RGB format.
+
+OWLEN will display an error message on startup if a custom theme has invalid colors.
+
+## Creating Custom Themes
+
+To create your own theme:
+
+1. Copy one of these files to `~/.config/owlen/themes/`
+2. Rename and modify the colors
+3. Set `theme = "your-theme-name"` in `~/.config/owlen/config.toml`
+4. Or use `:theme your-theme-name` in OWLEN to switch
+
+## Embedding in Binary
+
+These theme files are embedded into the OWLEN binary at compile time using Rust's `include_str!()` macro. This ensures they're always available, even if the files are deleted from disk.
+
+Custom themes placed in `~/.config/owlen/themes/` will override built-in themes with the same name.