Primary menu holds less text than in the non-Pro theme

Hi, I have been working on a website (www.iworkitout.nl) using the Catch Responsive theme and decided to upgrade to the Pro version. After having restored the background image and some other settings the webpage still does not look exactly like the way it did when i used the non-Pro version of the theme. The challenge is that in the non-Pro version all the menu items in the primary menu fit in on one line, but after upgrading to the Pro version of the theme the last menu item called "uitloggen" does no longer fit on the first line and so it shows up on the next line. As i wanted to experiment with font types, sizes and colours i thought there would be an easy way to change the font size so that all menu items would once again fit next to eachother, but i seem to be unable to change the font size. I tried to enter some CSS script in Appearance>Customizer>Theme Options>Custom CSS Options but couldn't find the right script to change the font size to a smaller one. Any help would be greatly appreciated. Thanks in advance! Andrew