Private property is a useful invention that serves human needs:
"Two things are competent to man in respect of exterior things. One is the power to procure and dispense them, and in this regard it is lawful for man to possess property. Moreover this is necessary to human life for three reasons. First because every man is more careful to procure what is for himself alone than that which is common to many or to all: since each one would shirk the labor and leave to another that which concerns the community, as happens where there is a great number of servants. Secondly, because human affairs are conducted in more orderly fashion if each man is charged with taking care of some particular thing himself, whereas there would be confusion if everyone had to look after any one thing indeterminately. Thirdly, because a more peaceful state is ensured to man if each one is contented with his own. Hence it is to be observed that quarrels arise more frequently where there is no division of the things possessed."
But it is some cosmically ordained, inviolable "right"; when it does not suit human needs, its rules need to be set aside:
"[I]f the need be so manifest and urgent, that it is evident that the present need must be remedied by whatever means be at hand (for instance when a person is in some imminent danger, and there is no other possible remedy), then it is lawful for a man to succor his own need by means of another’s property, by taking it either openly or secretly: nor is this properly speaking theft or robbery."
Note well: Aquinas is NOT saying "Sometimes it is OK to steal." He is saying, "In cases of great need, using what would normally be the private property of another is NOT stealing."