feat: color theme system for effects and UI

[HerbHall]

Description

Configurable color themes for transition effects and UI accents. Default is green (current), with popular terminal color presets and a random mode.

Color Presets

• Green (default) -- Matrix/hacker aesthetic
• Cyan -- Tron/cyberpunk
• Gold/Amber -- Retro terminal
• Red -- Warning/intense
• Purple -- Synthwave
• Blue -- Cool/calm
• White -- Clean/minimal
• Rainbow -- Cycles through all colors

Configuration

• theme_color: "green" -- primary accent color
• random_color: true -- random color per session or per transition
• random_color_pool: ["green", "cyan", "gold", "purple"] -- which colors to include in random rotation

Scope

Colors affect:

• Transition effects (rain color, dissolve color, etc.)
• Menu cursor and selection highlight
• Title text accent
• Tip/footer accent colors

Game-internal colors are NOT affected (each game has its own visual identity).

Research Needed

• Which terminal color values look best across dark/light themes?
• Should we detect terminal background and adjust automatically?
• Lipgloss adaptive colors vs manual light/dark palettes

Dependencies

• Depends on feat: user configuration system with CLI flags #41 (configuration system)