I am sorry to read this but I am almost certain that she will get the rest of the 20 days unless the judge has a very good day.
The sad part is that provoking a teenager to a fight is a guarantee that things will get out of hand. Teenagers cannot control themselves like we adults can. Their brains are not fully developed.
Second in legal terms assult is not theft. Assult means detention regardless if the victim is a family member or partly provoked the fight. That is also sad because the attacker (when we are talking minors) needs to be confronted with the victim and deliver an apology in the court room.
In some places they have created courts for teenagers, which actually works. It is the peer courts. I came to think about your post where you told of the first sentence when I saw a 3-part video (Part 1, Part 2, Part 3). This girl has to give a public apology in court and do some community service before her record is wiped clean. While she does that she educated the other kids in the court room and the good thing about a peer court is that they don't get a record if they do the things they are sentenced to.
What does happen in the juvie? Here in Denmark it is a job interview with biker gangs or some Osama wannabees fighting over the same drug market. I certainly hope that she will refrain from getting new friends or learn more tricks as the shoplifting tricks like russian bags (shoppingbags coated with tinfoil inside. I worked in the retail business and get the newsletters.)
I think that there is a way to keep her from getting the rest of the time juvie. You need to tell the judge that you will sent her to family far away for a period so she can therapy for something while avoiding ending up in further trouble. Maybe if you show that you are prepared to go very far, she can avoid juvie which would not benefit her at all. In fact once she is in there for a longer time, she will risk becoming institutionalized and then adult jail will be an environment she will function in far better than on the outside.
Try also to testify how the attack that got her the previous sentence was somehow provoked so the judge will learn that it was something out of character for her. If you don't the judge will think that he is facing a hardened criminal with assult and now shoplifting on her record and the sentence could very well be more than 20 days.
(I forgot. If the judge ask what she is supposed to get therapy for then claim that it is for Oppositional defiant disorder. It is something all teenagers suffers from.)