South Africa has 3 festivals in this guide, spread across 3 cities. They run in February, April and October, and festival-day temperatures range from 24°C (75°F) to 30°C (86°F), with nights down to 5°C (41°F). That range is the reason there is no single "South Africa festival outfit". The answer changes by month and by site.
1 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. 1 carries a high rain risk, which makes waterproof footwear a requirement rather than a precaution.
The ground matters as much as the sky. You will find city park, desert and open field, which puts waterproof boots 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, edm, amapiano and neon. Ultra South Africa is the largest of them at around 40,000 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.