Update JavaScript functionality in demo.js

[LandonMoss]

Improved skin changer functionality to support multiple themes and enhance theme switching behavior.
Enhanced the height calculation and scroll behavior for skin and setting lists to ensure a consistent user interface across different screen sizes.
Refactored code to handle window resize events efficiently and update the layout accordingly.
Updated Google Analytics integration to use the latest tracking ID and optimized tracking code for better performance.
Minor code cleanup and documentation improvements for better readability and maintainability.
Trying to reach out to admin to make these updated changes in demo.js file