Style menu

I wanted to know if it’s possible to style the menu with CSS. In particular I wanted to have menus that rolled up so that they didn’t fall off the bottom of the screen.

The menus have this behaviour for mobile devices (so I guess the code exists in the theme), how can I do this for other viewing platforms?

It would be nice to be able to manage the size of child menus as well so the font size could be smaller.

The gallery menu here illustrates the problem: https://gallery.heinrich.id.au