Well, the themes usually include anticipation. The main character is always waiting for the next sign or note from the secret admirer, which keeps the story exciting. There's also the theme of unexpected love. It often turns out that the secret admirer is someone the main character didn't expect, like a friend they've known for a long time but never considered in a romantic way. And, of course, there's the theme of vulnerability. The main character may feel vulnerable as they open up to this unknown admirer.
One common theme is the element of mystery. Since the admirer is secret, the protagonist spends time trying to figure out who it could be. Another theme is the slow - burn of the relationship. As the secret admirer reveals themselves bit by bit, the romantic tension builds gradually. Also, self - discovery often plays a part. The protagonist might learn more about themselves through this unique form of courtship.