Style refers to the way the text is written. How's that for vague?
So let's get more specific. Is the text simple? Grandiose? Wordy? Complex? How are the sentences structured? Does the narrative jump around in time and place? Does it include lots of vivid descriptions? While tone is about emotions, attitudes, and feelings, style is much more about the art and structure of the thing.
You can't talk about style without looking directly at the text. This is one of those times when textual evidence is your best friend. Don't believe us? Just ask Willy Wonka.