Navbar problem

Hello to all

I’m a beginner in website creation and I need help.
I would like to change the current navbar (photo 1) of my site and make it overlay my photo block (carousel) with a transparent gradient. On the current navigation block I can perfectly get what I want. Then to make the overlay I modify via a class the margin of my carousel top to -52.
The problem is that my navigation bar is not permanent and that it only appears if I scroll the page. How can I make it permanent ?

Thanks

picture one : actually

picture 2 : with navbar Ok but not appearing

picture 3 : with nabber like I want but only present when I scroll