I think that the "great" theme element might be a bit subjective. For my bet, I like the idea of maturation that is present in the poem. The notion of the child being the father of the man is a Romantic theme that is resurrected in the poem. Specific lines that bring this theme out would be the notion of being able to stand out in a crowd, following one's own voice, and ensuring that there is not a complete capitulation of identity to an individual's social setting. I think that the poem stresses this idea of being able to mature by being responsive to one's own set of core values which must remain permanent in a tide of continuing and impermanent social setting. I think that this is one of those themes that allows the poem to transcend times periods and settings.
Kipling's famous poem, 'If', is an expression of Victorian stoicism, illustrating the virtues of exemplary leadership and adorable manhood, righteous behaviour, idealisation of work, stoical detachment and the need to achieve the balanced middle way--'golden mean'.
Looked upon as the 'Prophet of British Imperialism', Kipling wrote on the virtues required for achieving the peak of moderation, virtues like leadership, hard labour, righteousness without self-righteousness, patience, detachment with determination, courage and, over and above, manhood.
Some of the major themes of the poem include value of leadership & attainment of manhood, righteous behaviour without being self-righteous, idealisation of hard work, detachment with determination, and the need to achieve golden mean.
Kipling's poem is primarily addressed to boys who are exhorted in the closing line to attain manhood. True to his strong Victorian mindset, the theme as underlined is characteristically patriarchal.
The poem exhorts the readers to be patient and forthright, but the poet wants this righteousness be shorn of any emulation of self-righteousness.
Ability to act is more important than the ability to philosophise. One should have very strong work-ethic. There is no wrong if one had dreams and thoughts, but such dreams and thoughts must not be masters, diffusing one from the path of pragmatic realisation of dreams.
Kipling advocates detachment from material success. The mind should be indifferent to success and failures of the material world.
The closing note of the poem is the vindication of the Middle Path, the golden mean'--the balance as the desired outcome of contradiction of opposites.