Final answer:
The theme highlighted by the quote is the discrepancy between social class and true gentility, with Mr. Gardiner embodying gentlemanlike qualities despite his lower social standing.
Step-by-step explanation:
The theme best supported by the quote provided is that of social class and gentility. The quote highlights the disparity between perceived manners or breeding and actual behaviour, noting that Mr. Gardiner, though involved in trade and not of the highest social rank, is in fact more gentlemanlike than his sister and more agreeable than the ladies of higher social standing at Netherfield might expect. This theme is prevalent in the literature of the time, which often explored the nuances of social rankings and behaviour.