Year around it is possible to travel by bus from Zurich to Pregrada, the bus goes via Zagreb, where passenger i most cases have to change bus, Click on the INFO icon in the timetable below to see operation dates of bus. The travel time for the 920 km long bus route from Zurich to Pregrada is about 15 hours.
If you plan also to travel back, (You can see return timetable here), you should buy a return ticket as this is cheaper then two single tickets.