Given that poems, like all texts, are subject to personal interpretation, the reader's response to the poem is how a theme is detected. In order to support one's denotation of a theme, one must have substantial textual and interpretative evidence to support the theme.
That being said, the idea of the theme speaking to the problems associated with the battle between technology (mankind) and nature can be justified.
The poem details the problems which may arise when nature and mankind meet. If the car had not been driving down the road, the doe would not have been hit by a car. Herein lies the main conflict: man verses nature.
One could ponder whether or not man has impacted nature in such a way as to destroy nature. In the poem, man wins. Not only has a car killed the doe, the speaker (in the end) kills the baby deer. In this circumstance, man is (by far) more powerful than nature.
The speaker, though, has a moment where he (assumptive based upon the gender of the author) stops and allows nature to listen to the decision he must make. Will the speaker try to save the baby deer or, will he end its life? In the end, the speaker chooses to end the life of the baby deer.
the theme is man vs nature