Romania has 3 festivals in this guide, spread across 3 cities. They run in July and August, and festival-day temperatures range from 28°C (82°F) to 29°C (84°F), with nights down to 15°C (59°F). That range is the reason there is no single "Romania festival outfit". The answer changes by month and by site.
Every one of them run 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. You will find beach, open field and arena grounds, which puts flat sandals you can rinse, plus trainers for the night stages at the top of the list for most of these events. 2 of the 3 have camping on site, so outfits have to survive living in a bag on the ground for several days.
Stylistically the crowd here leans rave, neon, costume and fantasy. UNTOLD is the largest of them at around 380,000 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.