Add more built-in themes

[AlexSkrypnyk]

Summary

Ship several more ready-made, name-selectable themes beyond the current Default theme.

Details

The theme architecture already resolves a theme name to an instance and lets a theme override styles and glyphs while inheriting the rest.

Add a small set of additional curated, high-contrast palettes selectable by name, so consumers get attractive options without writing a theme class.

Acceptance criteria

• At least three additional built-in themes are registered and selectable by ->theme('<name>').
• Each new theme is a self-contained theme class that overrides styles/glyphs and inherits the neutral base.
• Each renders correctly across all widgets and in both ANSI and no-ANSI output.
• Unknown theme names still fail loudly, with no silent fallback.
• Unit tests cover registration and resolution of each new theme.
• The new themes are listed in the README themes section.