Add dark mode toggle for functionality and styles

[Sadrita404]

Reference Issue

Overview

Added a day/night toggle switch right inside the 3D model box, letting users change the background and lighting in real time without messing up the rest of the page's styles.

Changes

• Isolated Theme Scoping: Restricted all background changes strictly to the #demo-container using localized CSS classes, ensuring the left side and other page elements remain completely unaffected.
• UI Transitions: Added simple CSS transitions to handle the colors, button borders, and icon rotation dynamically for a smooth, glitch-free click interaction.

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Before

Before.mp4

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After

After.mp4