I know that poetry can sometimes seem to be obscure, but that is because you are probably not reading it in context. In order to really understand the theme of a poem, you have to know something about the poet, probably something about his or her life, and something about the era in which he or she lived.
In addition to the above, you should know that there are only a few common literary motifs and your theme is going to be found somewhere among them.
The first set of themes includes: (1) coming of age, which includes the search for individual identity, loss of innocence, or growth from simply experiencing life itself; (2) the life cycle, which includes major events in life, such as birth, falling in love, growing old, and death; (3) the quest, which includes undertaking journeys that require overcoming insurmountable obstacles or completing impossible tasks in order to achieve a goal; and (4) the outcast motif, in which an individual is either isolated or excluded from society. This can be voluntary or involuntary isolation and the motives can be social, cultural, or physical.
However, be careful when choosing how you present your theme because your particular instructor may subscribe to common themes represented by "man vs. man," "man vs. Nature," "man vs. himself," or "man vs. society." In effect, know your instructor and follow his or her lead when choosing to identify a particular theme for a particular poem.