The Scarlet Ibis has a somber tone because you predict that something sad will end up happening to Doodle based on foreshadowing throughout the story.
S - Style
When the scarlet ibis is brought into the story, Doodle immediately has a connection with it because it was different like him. The ibis shows as a representation of Doodle to the readers. When the ibis dies, we're able to predict that the same might happen to Doodle.
T - Theme
In this part of the story, the narrator takes Doodle to the swamp for the first time and describes it as "the only beauty he knew." When they got there Doodle began to cry because it was so beautiful. It was one of their first real bonding moments.
The Scarlet Ibis seems to have been written as a narrative because it uses journalistic styles to help the reader understand the story in a better way.
I believe that the theme of the story is that even things that may be classified as "different" can have such meaning and can be beautiful. This is represented by the Scarlet Ibis and Doodle because they both end up meaning a lot to at least one other character.