next/prev button remove image

Hi there,
I changed the css and removed the images from the next and previous button.
but when the buttons are clicked, I still see the flashing of the images, any help on how to remove it would be really appreciated.
I’m working on this http://www.davidecossu.com/fashion-beauty/#jp-carousel-160

https://ww.wp.xz.cn/plugins/carousel-without-jetpack/