Fix main button on prod and background on dev

[shahar1]

Copilot Summary

This pull request improves the site's theme handling and header animation responsiveness to theme changes. The most significant updates are the addition of an early theme initialization script to prevent flashes of unstyled content (FOUC) and the introduction of a MutationObserver to detect and respond to theme changes in the header animation.

Theme Initialization and Responsiveness:

• Added an early theme initialization script to head-end.html to set the correct theme (light or dark) before page content loads, preventing FOUC and ensuring that animations use the right theme from the start.
• Updated headerAnimation.js to include a MutationObserver that listens for changes to the data-bs-theme attribute on the document element, ensuring the header animation stays in sync with theme changes.