I added a static link to a stylesheet and external javascript file in html_header.php, but the files do not show up in the header after a browser request.

how do I add external js and css to the <head> tag

I also need to preload images in the body tag as well.

I am adding a custom drop down menu that covers non zen cart areas of the website.

I searched for answers but could not find them.

P.S. this is my very first Zen Cart project and this is my first day on the job. So I may be searching for the wrong answers.

Thanks in advance.