Top Menu Bar overlaps Slider (Creative Slider module)

[cs071372]

Hi, I am looking to change my current slider to a full width slider using Creative slider module. However when I add the new slider the menu goes behind the slider. I have read the posts here and here however I am not sure where exactly to edit the css file to fix the z-index. Where in my httpsdocs I should look to fix that?

my url link is : https://opersis.gr/el/

 

P.S. The slider that appears at the moment works fine. The menu goes at the back of the slider when I am adding a new full width slider.
thanks in advance