feat: centralized color management system using CSS variables and Tailwind extension

[AnkittChauhan]

What does this PR do ?

This PR implements a centralized color management system as described in issue #449.
All hardcoded colors have been replaced with CSS custom properties and Tailwind color extensions.

Changes

• Added CSS variables to index.css
• Extended Tailwind config to reference CSS variables
• Added theme-aware utility functions in theme-classes.ts
• Updated StableDiffusionChat to use new utilities
• Removed redundant and conflicting class definitions

Benefits

• Single source of truth for colors
• Easier theme updates
• Improved maintainability and consistency

[Copilot]

Pull request overview

This PR addresses redundant and conflicting CSS class definitions in StableDiffusionChat.tsx as part of a broader centralized color management initiative. The change focuses on refactoring a single className that had duplicate background color classes.

Key Changes

• Replaced template literal string concatenation with the cn() utility function for better class merging
• Removed duplicate bg-TT-slate class that was causing conflicts
• Reorganized static classes to be outside the conditional expression for better maintainability

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.