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.

crates/owlen-core/src/lib.rs

@@ -12,6 +12,7 @@ pub mod provider;
 pub mod router;
 pub mod session;
 pub mod storage;
+pub mod theme;
 pub mod types;
 pub mod ui;
 pub mod wrap_cursor;
@@ -24,6 +25,7 @@ pub use model::*;
 pub use provider::*;
 pub use router::*;
 pub use session::*;
+pub use theme::*;
 
 /// Result type used throughout the OWLEN ecosystem
 pub type Result<T> = std::result::Result<T, Error>;