Theory:

The theme in a story is its underlying communication or 'big approach.' In other words, what significant idea about life is the author trying to communicate in the writing of a novel, play, short story, or poem? This knowledge, or concept, transforms cultural limitations. It is normally universal in nature. When a theme is universal, it touches on human existence, unmindful of race or language. It is what the poem means. Regularly, a piece of writing will have more than one theme.
 
How Does a Poet Develop Theme?
 
It really comes down to what the writer assumes about life. If a writer has a knowledge system or feels great about specific things and most people do, then those powerful life illustrations will be shown in his or her writing. For a work to remain, it needs wisdom and depth in writing indicates depth in the writer.
Theme of the poem "A Poison Tree": Rage and Suppressed Feeling
 
In the poem "A Poison Tree" the poet presents a debate against the suppression of anger. He is telling about the advantages of talking about anger, and the importance of keeping negative emotions within, the poem refers to the reader that the suppression of anger is implicitly threatening, leading only to more rage or even fury. The speaker offers two different situations to show the danger of crushing anger. Sincerity and honesty makes anger to disappear.