There are a number of themes in the book. I do not know which one you are specifically asking for, so I am going to list a couple of themes and some relevant quotes.
The theme of identity is in the book. The book is about teenagers and school. Identity and cliques have to be a huge part of the book. Spinneli highlights different social groups, kids struggling with where to fit in, and Stargirl. Stargirl just won't be placed into one group. She is her own person and empathizes well with everybody. The problem is that at first many other students think she is super weird. They don't appreciate her individuality.
"I'll tell you one thing," Kevin said as we joined the mob in the hallways, "she better be fake."
I asked him what he meant.
"I mean if she's real, she's in big trouble. How long do you think somebody who's really like that is going to last around here?"
This next quote is important, too.
We wanted to define her, to wrap her up as we did each other, but we could not seem to get past "weird" and "strange" and "goofy." Her ways knocked us off balance.
I'm not sure what to call the next theme. Importance of learning? Knowledge? Wisdom? Regardless, the thematic concept is shown by how Stargirl is able to teach people about themselves through her words and actions. She's not a teacher spouting forth instructions for kids to learn. She simply does, and people eventually learn from her, and they understand her better. What's interesting about this theme too is that Stargirl doesn't do all of the knowledge sharing on her own. Archie functions as a sort of translator and sage of sorts for her.
"You'll know her more by your questions than by her answers. Keep looking at her long enough. One day you might see someone you know"
This quote works, too.
"Like so many of Archie's words, they seemed not to enter through my ears but to settle on my skin, there to burrow like tiny eggs awaiting the rain of my maturity, when they would hatch and I at last would understand."