Sub-menu items hidden until mouseover?

Hi

I love this theme and have spent hours trying – and failing – to find another one similar because I am unable to hide my sub-menus. The nature of my site (mainly text) means I intend to have many sub-menus and so it would quickly look messy.

Is there any way to get this theme to hide the sub-menus until mouseover? Or is there a compatible plugin that will do the same without destroying the look of this theme?

Many thanks for any advice.