Charles Dickens wrote many novels, but the themes of those novels were often the same as one another and have relevance in any timeframe, even today and in the future. This is part of what makes a classic piece of literature a classic.
One very common theme is poverty. We see this theme in Oliver Twist, Hard Times, and even A Christmas Carol. Dickens was writing during the Industrial Revolution in London and the stark contrast between the wealthy and the poor was more prevalent than ever before. Lower classes worked very long, hard hours in jobs that often physically injured them and paid them very little. Families lived in abject poverty, were behind on bills, were on the brink of starvation, and all of that in spite of most members of the family working wherever they could, if they could. Those who could not pay their bills were put into debtor's prison (where they could not work) until their debt was paid off. While our world today does not have debtor's prisons and work conditions have clearly improved in most developed countries, there are many countries in the world where the conditions of the Industrial Revolution are still seen in many ways. We have modern day "sweatshops" for example, where workers are expected to put in excruciatingly long hours (sometimes 18-20 in a day), are paid very little, and have few other employment options to leave for. In developed countries, we still see poverty as a significant social issue and probably always will.
Another common theme is tension between the social classes. This theme appears in several of his novels, including Great Expectations, David Copperfield, and A Tale of Two Cities. Much like poverty created tension between the halves and the have-nots, money helped to re-enforce and create new social tensions. Prior to the Industrial Revolution and the capitalization of society, wealth was often most respected if one's family had what was called "old money." This mean the money had been in the family for a long time and the family had been of high class for generations. During the Industrial Revolution a middle class was able to actually appear and move up financially, but socially they were not still s respected as those with old money. The poor were simply still poor, but now looked down upon even more by those in the new middle class and the upper classes. We certainly still see this today. One prime example of this is in America where we have the poverty class, the middle class, the upper middle class, and the wealthy. Think about the stories on the "Top 1%" or "Top 2%" that the politicians are always talking about. They are referring to those top 1% of earners in the country who own the majority of property and wealth. These upper classes control a great deal of our economy, our government, and our policies. So, we are constantly affected by social class and there are often tensions between those classes as their individual needs clash.