It is the main message or idea the author is trying to convey to the reader. The theme is the author's opinion of the subject.
Almost, change illustrating with to illustrate. But otherwise it looks right to me.
The use of contractions and colloquial 'slang' implies a writer, or persona adopted by the writer, that doesn't have a great deal in the ways of education, he/she appears to be writing things as they are said rather than how we as an audience know them to be spelt. This can be shown by quoting any contraction or wherever there's an amendment to the text, for example "hender" instead of "hinder".
<span>The use of nature also implies the simpleness of the two characters, by using the stream imagery it gives quite a straightforward steady approach, more gentle than say a river but still pretty much in that ideal</span>
