Theming with CSS variables and dark mode

[parsapanahpoor]

• Issue Title: Improve theming support and dark mode customization
• Description:
  Enhance theming by exposing key colors and styles via CSS variables and making sure dark mode looks polished. Consumers should be able to easily align the JSON viewer with their app theme.
• Tasks:
  • Identify key style tokens (background, text, borders, selection, etc.).
  • Expose them as CSS variables or clear class hooks.
  • Ensure dark mode theme is consistent and readable.
  • Add examples for overriding styles from a parent app.
  • Update documentation with theming guidance.