Thanks a lot! Your reply really made my day. I'm actually just a programmer with no background in music, but I always passionate about cinematic music and have been observing how they did it for years. You see how those music in games, movies, animes, or whatever always enchant the mood in those medias? That's what I'm trying to do in this game. Maybe I actually have ADHD cause I've been thinking about music throughout my daily life a lot lol.
So when I make music for the game, it's either 1. I already have the general idea of music I want to make in my mind, even before the dialogue or story is written, or 2. The dialogue is already written, but the emotion of that scene need a change of mood, so I just daydream a bit and it kinda comes out naturally. I also have to experiment a bit when making the song in FL studio, like the combat music in this game was supposed to be simpler, but then I tried some sound that I thought would sound cool and it turned out even better!
For the color scheme, I figured that using only blue shade would give the game that "night feeling" and it also make sprite creation much easier, cause I don't have to worry about any other color (thanks Reddit for the advice about limited color palette). It also save me some development time.
I'm not sure how to give advice about music, but if you've never learn about it before, I recommend start learning how to play piano, cause music theory make the most sense on piano (FL Studio even use piano when you "draw" the note in a song.)