I need to write a composition on the theme of a poem?

It's fine. To me the woods seem a little more alluring and mysterious (the two mentions of "darkness" and the vague unease suggested by no one's seeing him, and the notion that the horse thinks there might a "mistake") than your account of the poem implies, but what you have is a good beginning. The speaker opts for responsibility ("promises") over surrendering to the woods--and that's a theme.