What is the theme in The Thistle and the Rose by Jean Plaidy?

The quest for power is a theme repeated in many aspects of this story, beginning with Margaret's excitement with the fact that she is about to become Queen of Scotland, though she does realize that her brother Henry is in a better position because he will be king in his own right without being dependent on marriage to another person. Another aspect of this theme is seen when Margaret shows her attraction for the Earl of Angus.