One common theme is loyalty. Mikami's unwavering loyalty to Light is often explored in different ways, like how far he would go to protect Light's plans. Another theme is power dynamics. Since Light has a certain hold over Mikami, fanfictions often play with this aspect.
One popular 'Light x Mikami' fanfiction is 'The Unseen Bond'. It delves into their complex relationship and how they might interact in different scenarios not shown in the original work. It's really interesting to see the author's take on their dynamic.
One common theme is power dynamics. Light is often portrayed as having a dominant role, given his intelligence and the way he manipulates situations in the original story. Mikami, on the other hand, is shown as being more submissive to Light's will.
Another place to look could be FanFiction.net. However, the content there might be a bit more spread out. You'll need to use the search function and specifically look for works related to Light and Mikami in the yaoi genre. Keep in mind that the quality can vary greatly from one story to another.
Since I'm not familiar with this 'yua mikami love story', it's hard to say what the plot is. Maybe it's about a love journey between Yua Mikami and someone else, but without more information, it's just speculation.
I'm not sure specifically as it could be a rather unique or niche light novel. It might involve a story where a cube has some sort of cursed or curious properties. Maybe the cube is an object of mystery that brings strange events to the characters in the story.
One plotline could be a love triangle. The reader might be initially drawn to Light's charm and his seemingly noble goals as Kira, but as they get to know L better, they start to see his genius and his dedication to justice. This creates a conflict within the reader as they have feelings for both. Meanwhile, L and Light are also competing for the reader's affection while still being locked in their battle over the Kira case.
The complete collection of Ghost Blows Out the Light was called the Ghost Blows Out the Light series. The series was a story about tomb raiding and exploration from China. It included three main works: Ghost Blows Out the Light, Ancient City of Jingjue, and Nine-story Demon Tower. The series of novels had been adapted into TV series, movies, games and many other forms, which were loved and sought after by readers.