Stealing is certainly a recurring motif and dominant theme in Ian Serraillier's The Silver Sword. In addition, Jan's stealing is often juxtaposed against the Balicki children's behavior and beliefs.
Early in the book, after both their father and mother have been taken from them, the Balicki children are forced to fend for themselves. They find a basement in a bombed out home they can use as shelter and have to find food by any means possible. Edek, the 11-year-old brother, is afraid to sign up for a ration card because he knows he'll be sent to a German labor camp for having shot at a Nazi if he does. Instead, they went to soup kitchens whenever possible, begged from convents, and sometimes "stole from the Nazis or scrounged in their garbage bins," but most importantly, the Balicki children made it a point of personal principle to never steal from their own kind (p. 43). As the narrator phrases it, "They saw nothing wrong in stealing from their enemies, but they were careful never to steal from their own people" (p. 44). Their perspective can certainly be justified: Stealing from the enemy weakens the enemy, but stealing from your own allies only weakens your own forces. In times of war, society needs all the strength it can get to battle against the enemy, and strength only comes from unity, not disharmony.
In contrast, Jan, a child completely alone, steals any chance he gets; he even picks the pockets of the Balicki children's father, Joseph, when Jan meets him early in the book. As Jan sees it, "The Nazis stole everything from our country and left us with nothing," and now they were left to fend for themselves by any means possible (p. 128). However, when he is caught stealing by Captain Green, the captain echoes the Balicki children's sentiment: It is absolutely necessary for society to adhere to the biblical commandment "Thou shalt not steal" otherwise "everything will go to pieces" (p. 129).
Hence, it can be argued that author Serraillier is trying to show that though desperation makes stealing seem justified, it's important to remember the needs of society as a whole. If you steal from your own kind as well as your enemy, you only weaken the strength of your own forces against the enemy.