One common theme is possessiveness. Since Todoroki is yandere, he often wants to keep Deku all to himself. Another theme is the struggle between Deku's own agency and Todoroki's overbearing love. For example, Deku might want to pursue his hero career and have other friends, but Todoroki's yandere nature tries to limit that. Also, there's the theme of secrecy. Todoroki may try to hide his yandere tendencies from Deku and others at first.
Sure. There's a fic called 'Yandere Shoto's Desire'. It shows how Todoroki, with his yandere nature, would go to great lengths to protect Deku from any perceived threats, even if those threats are just Deku's other friends. It really delves into the darker side of his love for Deku. Also, 'Dark Love of Todoroki' is a fic that has Todoroki's yandere side emerging slowly as he falls deeper in love with Deku, leading to some very interesting and somewhat disturbing situations.
His friends would probably take care of him. Midoriya would be very eager to help, using his kind-hearted nature. He would make sure Todoroki is fed and comfortable.
One idea could be that all his friends at UA plan a surprise birthday party for him. They decorate the common room with his favorite colors, blue and red. Another plot could be Todoroki spending his birthday with his family, trying to reconcile with his father on this special day. Maybe he receives a heartfelt gift from his mother that has a deep emotional significance.
In yandere x reader fanfics, yanderes usually display intense emotions. Their love for the reader is not the normal kind; it's more of an obsessive love. They may have a dual personality where they can be extremely sweet and loving one moment, but if they feel threatened in their relationship with the reader, they can turn very dangerous and menacing.