Adding custom style problem

Good morning, adding a custom style through the custom style tab is not possible because I don’t see that tab. What could be the problem?
I only want to center the slideshow with the following code but through a normal css adjustment it does not work.

.slideshow_container_style-light .slideshow_container {
    margin: 0 auto!important; /* This centers the slideshow, if the slideshow has a maximum width. */
}

Thanks!