[Plugin: NextGen Gallery] Gallery not showing as it should

This is my site: https://www.princeslick.com/?page_id=28.

The theme was made by myself, and it does not support the get_header function, so I decided to insert the links myself for js and css files.

For some strange reason, when I click on the thumbnails, a “slideshow” opens up some 500px below my footer, instead of the usual popup. Can somebody help me see what’s wrong?