Site is working well and smooth. Design is a bit outdated. There's nothing wrong. Just lack of surprises. Spice it up!

Suggestion:

#1: For example on this page(http://www.zcgo.hr/o-nama) where you have to deliver long messaging, you can break them up into different paragraphs. Key points or key paragraph can be styled into different font sizes or colours so user will always stay in focus.

#2: Different header images on each page. To avoid user dropping off your site, one of the solutions is to include rich media or any interactive elements. Use better imagery. Maybe embed videos that related to your service.

Hope my words can help you. Thanks :)