[FEATURE] Implement New UI Based on Figma Design Using Pure CSS (No TailwindCSS)

### Feature Description

We are redesigning the UI of our website based on a new Figma design. Do not use TailwindCSS for this implementation.

Design: https://www.figma.com/design/gAIj0zUS9FicOQvpERBKMu/Old-School-Games?node-id=0-1&p=f

Style: https://www.figma.com/design/gAIj0zUS9FicOQvpERBKMu/Old-School-Games?node-id=16-5&p=f

Tasks:
- [ ] Review and understand the Figma layout and components
- [ ] Replace existing UI with new components and structure
- [ ] Style all elements using Pure CSS
- [ ] Ensure layout responsiveness (mobile, tablet, desktop)
- [ ] Match fonts, colors, spacing, and visual hierarchy from the Figma
- [ ] Remove all TailwindCSS utility classes (if any)
- [ ] Test across major browsers (Chrome, Firefox, Safari, Edge)

Change the design theme from classic game to Brain train game