Feature Request: Add Example for Custom Theming

[sidfakenews986]

Feature Request

It would be great to include an example in the README that demonstrates how to implement custom theming with tanstack-theme-kit. Currently, the README provides a clear usage guide for enabling dark mode, but new users might benefit from seeing how to define and apply their own themes.

Suggested Implementation:

• Add a section titled Custom Theming to the documentation.
• Include a code example showing how to define custom CSS variables for themes and switch between them using the useTheme hook with the ThemeProvider.
• Optionally, document how to persist custom themes across sessions.

This addition would enhance usability and help users get more out of the library.