Good vs. Evil - the author describes Grendel and his mother as dreaded monsters. Beowulf; who overcomes them through different approaches, is describes as the quintessential human being, who is honest, honorable, loyal e.t.c. albeit with quasi-superhuman powers.
The universal themes covered by the description of battles between Beowulf and Grendel and between Grendel's mother are:
First of all, Beowulf is an epic story about a warrior that fights for different causes but that all of them have moral value fundaments. In the first place, he tells the story of how his father was protected by a king and that because of that Beowulf owes loyalty to that man. Also, Beowulf fights for a king and follow his commands. But that reputation makes him decide between the presentation of Grendel's head or the king. However, after that Beowulf is tempted by Grendel's mother and makes a deal with her for ambition, pride, and reputation. So, after many years he has to face the son he had with Grendel's mother and he regrets his actions. Envy is also represented in the story by Grendel and how he is envious about men because god blessed them but not him.
The battle of good and evil. Beowulf is a young warrior who wants to stop the terror of monsters. He is a paragon of good, just like Grendel's mother is a paragon of evil. She is an implacable force of evil, who tries to exterminate the hero who has dared challenge her.
Courage and self-sacrifice. Beowulf has many reasons to fear the furious monster who wants to avenge her son's death. Nevertheless, he enters the battle, fully aware that he is risking his life. His courage is one of his heroic traits, which separate him from other humans.
Magic and supernatural. First, there's the fact that Grendel's mother is living in a lake. Then, there's the ancient sword that seems to be the only way to kill this monster, even though it melts down after Beowulf has decapitated Grendel's corpse with it. Then, there's the fact that Beowulf somehow manages to survive under water, even though he is only a human.
hope i helped