An old performer's adage proclaims, "Always leave 'em laughing."  When it comes to theme, this truism could be paraphrased as "Always leave 'em feeling."  Theme is an emotional argument, and the best way to make such an argument is by involving the reader/audience at a personal level.

Since the message issue is the human quality that your story is about, there is already a built-in level of attachment to your reader/audience.  But people don't like to look at themselves critically.  The trick, then, is to find a way to involve their emotions without pointing a finger at them.

Quick Tip - You can draw your readers/audience into an emotional bond with your message issue by devising illustrations that show the impact of that issue on characters they have come to care about, rather than preaching your message directly.  When it comes to theme, show, don't tell.