Without much hesitation, I can safely say the theme of this story is fairly simple: adventure in the American Old West. It is actually the very first of the Sackett novels (named from the main character, Tyrel Sackett, who is eighteen years old in 1866 when the story starts). The two Sackett brothers break the law back east, decide to move west, and fall in love as they travel.
The theme of adventure runs through absolutely every episodic thread of the novel. There are cattle drives that present much danger to both Tyrel and Orrin. But by far, the main thread here is the adventure of love. Orrin falls in love with a selfish gal named Laura, but Tyrel falls in love with a compassionate Spanish beauty named Drusilla. The two brothers, with their loves in tow, traverse the gold rush in Idaho. They head through Abilene, Texas, Kansas, and finally Santa Fe, New Mexico.
Violence is an evil thing, but when the guns are all in the hands of the men without respect for human rights, then men are really in trouble.
Therefore, as you can see, the brothers are drawn into preventing violence through the law. All throughout, they hear from their mother and their older brother who is fighting the Sioux Indians in the North.
In conclusion, it is important to note the irony in the brothers becoming "law men" at the end of the story. Remember, the reason why they left the East was because they broke the law (through murder of a man trying to shoot Orrin, who was unarmed).