Extension JS works in debug but not in prod

Hi homolworks  It is not recommended to change the predefined layout in SharePoint, but if it is really needed in your case, you can try assigning a CSS file in the Application Customizer. 

    //set the css file url
    const cssGlobalUrl = 'https://contoso.sharepoint.com/Styles/Main.css';

    //get the head element
    const element = document.getElementsByTagName('head')[0] || document.documentElement;
    // assign CSS file url to the link element
    const linkElement = document.createElement('link');
    //set CSs Version by time
    linkElement.href = `${cssGlobalUrl}`;
    linkElement.rel = 'stylesheet';
    linkElement.type = 'text/css';

    element.insertAdjacentElement('beforeend', linkElement)

Let me know if it works in your case. In your current approach, the elements might not have been rendered yet when you check them in the code