Hi verkeerenvorm,
1. Make CSS formatting for how you like to make the top menu look like.
add it to your style sheet. Then make sure remember the name.
2. Apply to the module position via the module manager that is holding the
top menu.
see this link it may help its about tabs but will give an Idea.
http://www.m65.net/articles-joomla!-joomla-template-kits-tutorials-adding-a-tab-menu-50.html
in first picture you see the menu class add your CSS name to that field.
it will then render your formatting.
Cheers,
Brian