Add cyan accent color to enhance website aesthetics

This update introduces a vibrant cyan accent color alongside the existing
blue, creating a more visually engaging design while maintaining the clean,
minimalist aesthetic.

Changes:
- Define custom cyan and blue accent colors in global.css using @theme
- Add gradient effect (blue to cyan) on h1 and h2 headings
- Add cyan accent border on code blocks for visual distinction
- Update Button component with cyan border on hover/active states
- Add gradient indicator for active navigation items
- Add colorful hover states to theme toggle icons
- Include smooth color transitions for improved UX

Both light and dark modes are fully supported with appropriate color variants.

package-lock.json

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