Bernard Herrmann wanted to find a way to express both the vastness of Kane's power and the innocence of his childhood and the sled that represents it.
In fact, he makes such close connections between his music and ol' Rosebud that it actually serves as a sort of clue as to what Kane means when he uses that word. Overall, it's fair to the say that the score is pretty ominous in its beginning, since it is meant to reflect the total isolation and darkness of Kane's home in Xanadu.
Overall, though, the score is a lot about the opposing moods of glorious victory and shattering darkness. Which kind of makes sense, really, since that's what 99% of Kane's life is: a pretty intense roller coaster of highs and lows.