The most common themes are love against all odds. Given their circumstances as superheroes and the complex world they live in, their relationship is often portrayed as something that has to overcome many obstacles. Another theme is the discovery of self - through the relationship. They learn more about themselves as they fall in love with each other. And there's the theme of protection. Nightwing often wants to protect Robin, and vice versa, which adds a layer of depth to their relationship. There's also the theme of acceptance, as they may have to accept each other's flaws and the non - traditional nature of their relationship in the context of the superhero world.
One common theme is the struggle between their hero identities and their personal feelings. They are both vigilantes in Gotham, and often they have to put their duties first, which creates tension in their relationship. Another theme is the idea of mentorship turned into something more. Since Nightwing was once Robin, there's a connection there that can develop into a romantic one. And of course, there's the theme of secrecy, as they may not want to expose their relationship to the rest of the Bat - family or the public.
Common themes include the exploration of their emotional vulnerability. In many 'nightwing x robin' yaoi fanfictions, they show each other sides that they don't show to others. There's also the theme of growth, both as individuals and as a couple. They face challenges together that help them mature. Additionally, loyalty is a big theme. Their loyalty to each other is tested in different situations, whether it's against villains or due to misunderstandings within their relationship. It's also common to see themes related to Gotham itself, like how the dark and dangerous city affects their relationship.