One common theme is friendship. Their deep and unwavering friendship is often explored, like how they would always have each other's backs. Another is adventure, as they were both known for being daring at Hogwarts.
Common themes include camaraderie. They were part of the Marauders, so stories often show their interactions within the group. There's also the theme of growing up. How they changed from carefree students to more responsible adults involved in the fight against Voldemort. And of course, the theme of sacrifice, as they were both willing to sacrifice a great deal for their friends and the greater good.
Well, loyalty is a big one. They were extremely loyal to each other in the books, and fanfiction often delves deeper into this aspect. Also, mischief - making. They were quite the pranksters, so many stories feature their elaborate pranks and the consequences. And family also comes up sometimes, like how their own family situations affected their relationship.