Encapsulate game palettes in theme styles

[bashmohandes]

Motivation

• Remove per-game hard-coded palettes and centralize game-rendering colors in theme styles so new themes can be added without modifying game code.
• Provide a stable contract for canvas games to consume theme-owned tokens rather than selecting a theme or embedding light/dark palettes.

Description

• Move Battle Tanks palette values into theme CSS as --battle-* custom properties in styles/modern-game.css and scope them for the game via .game-battle-tanks selectors.
• Replace the in-code palette lookup in battle-tanks/scripts/app.js with a computed-style adapter that reads battle tokens from getComputedStyle(arena) and builds arenaColors dynamically.
• Document the contract in docs/adr/0008-experience-theming-system.md stating that theme styles expose game rendering tokens and canvas games must read computed properties.
• Add a regression test in tests/appearance-system.test.js to assert the presence of --battle-* tokens and ensure the Battle Tanks script reads computed styles and no longer contains the embedded palettes table.

Testing

• Ran npm run check which completed without runtime syntax errors for the modified scripts and modules.
• Ran the full test suite with npm test and the focused tests with npm test -- --test-name-pattern='theme styles own|alternative themes', and all targeted tests passed.

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Codex Task