I would like to make a menu that when opening ‘pushes’ the content that’s underneath down.
An example of this can be found here: http://www.bsr-lorraine-blouin.nl (if you click on the hamburger-menu-button it ‘pushes’ the text underneath downwards).
The site you have used for your example is what is called “fluid” - elements are relative to one another, whereas Sparkle uses a “fixed” layout - elements are “fixed” so other elements can’t move them from their placed spot.