Yes, of course:Using the Theme Editor :1. You click on the item you want to change on the left side of the screen
2. On the lower button of the screen you find the correct description : f.e. HTML>BODY>DIV>DIV.Mobile_view>DIV.body>UL.menu_list>LI.menu>SPAN
3. Using this description, you add a new rule on the right side of the screen (including the >).To change the Font-size use font-size : XXpx
Hope that was clear.....
For best results, use the Firefox browser..