What are three mains themes of Rerum Novarum?

Rerum Novarum (literally "On the new things") was an encyclical issued by Pope Leo XIII in 1891. The alternative title of the document, Rights and Duties of Capital and Labor, gives some idea of the content and tenor of this significant affirmation of Catholic social doctrine. By the late 19th century, free-market capitalism had advanced considerably in the West, creating massive economic change and significant social upheaval. Given the radical changes taking place, it was inevitable that the Pope would make a contribution at some point to the ongoing debate about the effects of this rapidly developing economic system.
The first main theme is that of the dignity of the human person. Pope Leo shows an acute awareness of the appalling suffering and poverty that free-market capitalism often creates along with the enormous wealth it generates. To that end, workers must expect to be treated fairly by their employers, given adequate leisure time and opportunities for acts of worship. Workers must receive a fair day's pay for a fair day's work and not be expected to labor in unsafe conditions. At the same time, the Pope asserts traditional Catholic teaching with regard to the role of women, saying that they should ideally work at home. This is a recognition by Leo of the immense challenges posed by capitalism to the traditional family unit, with more women being forced to work in order to contribute to household upkeep.
Society must be organized on the basis of the common good. Capital and labor both have their own interests, but it is the role of the state to ensure that those interests are to some degree reconciled as part of a more united society, one that respects the innate value of each individual. Pope Leo is trying to move beyond an antagonistic model of class relationships, recognizing as he does that such antagonism creates opportunities for socialist agitators with their profound hostility to religion and traditional social structures.
Equal value is accorded in the encyclical to rights and duties. For every right, there is a corresponding duty. Workers, for example, have the right to fair wages and conditions, but they also owe a duty to their employers. By the same token, employers have the right to own private property, but also a duty to treat their workers fairly and with dignity. Once again, we see the Pope trying to steer a middle course between what he sees as the extremes of unfettered capitalism and radical socialism.