Thank you for your help. It was not quite what I needed, but it helped me to find the lines I needed to edit in the css.
Your Code deactivates the scaling in the lightbox effect, i.e. the item-mask does not scale when the mouse hovers over items.
What I needed was to deactivate the transparency effect, that dims elements, which are not in the focus of the mouse. I managed to deactivate this by using the following code that deactivates the transparency effect.
.overlay-shape:hover li, .no-shape:hover li, .related-items:hover li {
opacity: 1;
-moz-opacity: 1;
filter:alpha(opacity=100);
}
Thanks again ๐