South Africa defeated the host nation 26-19 in La Roche-sur-Yon on Thursday in their last group match, securing top spot in Pool A.
"We knew that the French would be passionate and very committed because they had so much to play for," Theron said.
"They were lifted by a fantastic crowd and gave it their all, but I am proud of the effort by each of my players because the win is what matters most, and we deserved it in the end.
"Our guys stood firm, made some vital tackles and in the end we clinched it with a great try by Cheslin Kolbe."
The Junior Springboks twice trailed in the match -- and were matched for tries at three-all -- but they showed tremendous defensive character to hold off the host nation.
Theron was not pleased with the efforts in the scrums, and insisted his team would need to work hard to correct their mistakes ahead of their semi-final against Wales in Vannes on Tuesday.
"We missed Luhan (de Bruin, suspended prop) and there were some puzzling calls made against us," he said.
"The coaches will go and have a look at the video and try to figure out what went wrong.
"We are in the semi-finals and will give it our best effort in the next match."
Junior Springbok captain Ruan Steenkamp said the ferocious French forward pack made it difficult for them to gain momentum.
"They disrupted us from the start and forced us into mistakes," Steenkamp said.
"We could not get the momentum that we wanted but we showed a lot of bravery and heart out there.
"We played much better towards the end, especially during the last 10 minutes, and I am just relieved that we managed to beat them because they are a dangerous team.
"We will now regroup and prepare for the semi-final."