The book of Revelation is one of the most difficult books in the Bible. In order to understand it, you need to know a lot of the book of Daniel, which is another very difficult book. As for themes, there are a few main ones.
First, from a structural point of view, there is strong evidence that the book repeats itself in different ways, but it always ends in the same way, with the victory of Christ over all. Revelation 19 is emblematic. Jesus is mounted on a white steed and will come to judge and bring victory. Hence, the victory of God is the main theme. This is why the book ends with a vision of a new heaven and new earth.
Second, another theme is counterfeiting. Let me give one example. In Revelation 13 there is a beast, dragon, and false prophet. The dragon gives power to the beast, one of whose many heads experiences a resurrection, and on account of this, the prophet tells the world to worship him. This is a counterfeit trinity. In other words, this mimics the trinity. The Father gives authority to the son, who experiences a death and resurrection, and the Holy Spirit illumines people's eyes to see the work of the son.
A third theme is the importance of perseverance. The book calls Christians to persevere through hardships and persecution. This makes sense, because the book was probably written during the time of the Roman emperor Diocletian, who persecuted Christians.