One of the themes of Hiroshima is that we can use a powerful new technology without fully understanding it. When the atom bomb was first dropped on Hiroshima, the American public, rejoicing that the war was over, did not fully grasp how deadly nuclear power was. The book alerted the American people to the terrible and destructive potential of this weapon by describing the gruesome effects of the bomb on those who died. The theme of understanding the technology we are adopting is even more relevant today than in the 1940s, as today we gain knowledge at more and more dizzying speeds. As we develop the ability to alter genes, for example, we need to stop and think about the long-term impact of what we are doing when we change genetic structures that have evolved over millions of years.
The book also has a theme that an individual never knows what is going to happen. The six characters in the book have no inkling of the way their lives are going to change in a literal flash. They wake up on August 6th, 1945, expecting an ordinary day, not a nuclear holocaust. As human beings, we all are subject to this possibility of sudden catastrophic change.
Finally, another theme is survivor's guilt. The six who survive have to deal the wrenching question of why they survived when so many others died, and to help others who survived deal with the psychological devastation of the atom bombing. In the wider world too, especially with more mass shootings and other upticks in violence, everyone has to be prepared for the idea that they might survive a tragedy that seems to have randomly taken the lives of people equally deserving.