Decay and death
Robert's life is full of pictures of failure, decay, and death. Instead of being with Celia, whom he loves, he impregnates someone else, leaving Celia in the king's possession. Then, his baby with Katherine dies. This decay signals that he is separated from his center, psychologically. In the end, he wishes to go back to his royal life so badly that he afflicts himself with real disease by being so unhappy. In the end, we are left to wonder whether the man is "restored" to his wealth, or whether he is having fever dreams in his comatose state.
Romantic loneliness
The issue of family is practically defiled by the characters in the book, and it's no wonder when the sexual misbehaviors of the king have their effect. When Robert marries Celia, the marriage is a sham, and the hollowness makes him really fall for Celia, whom he cannot have though she is already his literal wife. This inversion of marriage shows Robert's undeniable struggle with loneliness. Then Robert effects Katherine with the same sense of emptiness and rejection. He impregnates her on a whim, then when the baby dies, he just walks away from her, leaving her to deal with that horrifying tragedy alone.
Religion, God, and authority
The relationship Robert has to the king is very similar to Pearce's experience of God as a Quaker. Also, the king makes Robert marry Celia (violating the sacrament), so that the church doesn't catch him in an affair. The issue of authority is on full display, and when Robert spends (what could be) his last days working himself into a fever dream, he is imagining restoration to his place as a royal benefactor (not unlike the Prodigal Son). These are purposefully religious themes, and they balance the novel's attention to hopelessness and loneliness.
