One common theme is self - discovery. Since Zenitsu has a complex personality, many fanfics focus on him finding out more about himself, his powers, and his place in the world. Another theme is friendship. He often forms strong bonds with other characters in fanfics, like Tanjiro and Inosuke.
Definitely not. In the manga, there's no such plotline of Zenitsu and Nezuko getting married. Their relationship is more focused on being comrades in the fight against demons rather than a romantic one leading to marriage.
The Scream was a novel written by Lu Xun. It was about a small figure who was oppressed by society. In desperation, he let out a cry to attract the attention of society and finally obtained freedom and dignity. This novel was known as a classic of modern Chinese literature for its strong ideology and artistry.