Serbia has 1 festival in this guide, spread across 1 city. They run in July, and festival-day temperatures range from 31°C (88°F) to 31°C (88°F), with nights down to 18°C (64°F). That range is the reason there is no single "Serbia festival outfit". The answer changes by month and by site.
Every one of them runs in real heat (28°C or more), where loose natural fabrics beat coverage and a brimmed hat does more than any garment.
The ground matters as much as the sky. Every festival here is on city streets, which puts broken-in trainers or a low boot at the top of the list for most of these events. 1 of the 1 has camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans rave, techno, alt and camping. EXIT Festival is the largest of them at around 200,000 people, and it sets the tone the smaller ones react to. None of that is a dress code. It is what you will see, which is worth knowing if you would rather blend in or deliberately not.