Question

I know that many authors intentionally use polysyndeton (extraneous conjunctions) to create his/her own unique syntax, but how does this unique sentence structure contribute to tone and theme?

Answer 1

 Sentence structure and grammar convey tone and theme through delivery. It's the individual style of the speaker's perspective that colors how they see and convey the world and the messages and the tone. 

Example: Complete sentences can project a formal tone (attitude/atmosphere) whereas fragmented sentences might imply the opposite. Similarly, word choice helps define the tone and theme. 

Consider:  
-- "I'm going to die, mother. There is nothing that will change this." and  
-- "Momma. I'm go'n die. Ain't no one gonna change that."  

The first might imply an educated speaker, or a person who has come to terms with their mortality. It may even project a bitterness toward death in theme. They seem pragmatic about their impending death. The second suggests an uneducated (lower class) speaker and may project other emotions aside from acceptance of their death. There's an underlying emotion implied through the speaker's voice/personality and how they phrased their words, (perhaps the speaker is imploring their mother to accept their death too. Something beyond the words are implied because of *how* the speaker phrased the words). 

Similarly, if you're short/impatient with a person, your sentences may be clipped (No. Thank you.). But if you're more patient, you may take the time to offer an explanation to alleviate their worries, (No, thank you. I'm tired tonight. Long day at work.).  

Poor grammar (ie: fragmented sentences, run-on sentence structures, improper word usage, incomplete prepositions/clauses/sentences...etc), can denote many different things about the character/speaker to help project their style and how they interpret their experiences.