Feature: Implement Multi-Theme Support for Pong Game.

### **Feature: Implement Multi-Theme Support for Pong Game**

**Goal:**
Enhance the visual appeal and user customization of the Pong game by introducing a multi-theme system. This feature will allow players to select different visual styles or "themes" for the game's elements (e.g., background, paddles, ball, score display), offering a personalized and fresh experience.

**Context:**
This feature builds directly upon the fantastic visual enhancements introduced in PR #8 (Animated Gradients, Glassmorphism, Glow Effects). The goal is to make these beautiful visuals switchable, allowing users to choose from a set of predefined themes.

**Proposed Functionality:**

1. **Theme Selection UI:**
* Add a simple UI element (e.g., a dropdown menu, a set of buttons, or a small theme selector on the start/settings screen) that allows players to choose a theme.
* Consider adding a "Default" theme, and at least 2-3 new distinct themes (e.g., "Neon Retro," "Dark Mode," "Minimalist," "Ocean Blue").

2. **Dynamic Theme Application:**
* When a theme is selected, the game's visual elements should update dynamically without requiring a page reload.

**Implementation Ideas (Suggestions - contributors can propose alternatives):**

* **CSS Variables:** Define theme-specific colors, gradients, and other styles using CSS custom properties (variables). When a theme is selected, update the values of these variables via JavaScript. This is generally the cleanest approach for theme switching.
* **CSS Classes:** Apply a theme-specific class to the `` or a main container element (e.g., ``). CSS rules can then target elements based on this class.
* **Structured Styling:** Organize existing and new CSS to support theme variations (e.g., `style.css`, `themes.css`).

**Expected Deliverables:**

* A functional theme selection UI.
* At least 3 distinct visual themes implemented (including the current default style).
* Code that dynamically applies the selected theme.
* Updates to relevant files (`index.html`, `style.css`, `script.js` or a new `theme.js` if appropriate).
* Clear and commented code.

**Relevant Files (Potential areas for change):**

* `index.html` (for UI elements)
* `style.css` (for theme-specific styles, potentially using CSS variables)
* `script.js` (for handling UI interaction and applying themes, or a new `theme.js` module)
* Possibly `gameStates.js` if theme affects game state.