Update extra.js for auto theme

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved theme application process in the Ultralytics handbook documentation, simplifying the code and enhancing user experience.

📊 Key Changes

• Consolidated functions for applying themes, reducing redundancy.
• Simplified logic to determine and apply user-preferred themes.
• Added event listeners for dynamic theme changes and refactored code to streamline theme management.
• Enhanced support for multiple languages by improving language code handling in the theme process.

🎯 Purpose & Impact

• Efficiency: Refactoring the code makes maintenance easier and reduces potential errors.
• User Experience: Ensures seamless application of user-preferred themes, enhancing the interface's adaptability to user preferences.
• Internationalization: Better handling of language-specific settings supports a broader, global audience more effectively.

[UltralyticsAssistant]

👋 Hello @glenn-jocher, thank you for submitting an ultralytics/handbook 🚀 PR! This is an automated response to confirm that we've received your pull request. An Ultralytics engineer will review it soon. In the meantime, please check out the following checklist to ensure a smooth collaboration:

• ✅ Define a Purpose: Ensure the purpose of your modifications is clearly articulated in the PR description, and link to any relevant issues. Make sure your commit messages are clear, concise, and follow the project's conventions.
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• ✅ Update Documentation: Reflect any new or altered features in the relevant documentation.
• ✅ Add Tests: Where needed, add or modify tests to verify your changes, and ensure all tests pass.
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