Introduce predefined themes and style palettes #135
Description
Problem: Users must manually chain colors/styles for common patterns (error, warning, success), leading to repetition.
Why it matters: Themes improve consistency and accessibility (e.g., high-contrast mode).
Proposed solution: Add a Theme dataclass with named palettes and a console.theme('error')("msg") shortcut that internally sets fore/back/style. Allow loading custom themes from dict or file.
Notes: Ensure thread safety if themes are switched globally; prefer per-instance themes.