As I cannot edit the post, here an important add-on: turned out that this is only half the success story. So what you need to do, is to do the following:
#SLB-Wrapper #SLB-Contenedor{position:relative !important;}
Using this property in combination with !important, leads to ignoring the element entries in the div tag. These element entries are edited in
jquery.sexylightbox.min.js
and during my many experiments I changed here something (replaced an "absolute" value by "relative" - The problem is then, that the shadow in the background covering the whole screen is only covering the bottom of the screen. So do not do this! Leave jquery.sexylightbox.min.js as it is).
This is where I found the trick - remember to clear browser's cache.
css-tricks.com/override-inline-styles-with-css/
Works now on Firefox, Chrome and Safari.