Twenty Fifteen – Expand on menu item instead to the dropdown-arrow

late-edit: tl;dr: How can I add accordian navigation to twenty fifteen theme?

Hi! Been working on a site for the past few days straight but seem to have hit a wall here. Basically, the behaviour I wish for, is that the user can click on the whole menu item instead of only only the arrow-symbol next to it in order to open the sub-items.

Can I create a button with text? And if so, how do I even manage to save it? All I see are php and css files, but when I open the site with firebug I find way more understandable (to me) html code.

Thanks for your time.

edit: This is called ‘Accordion’.