There's an obvious play on words here: moon/sun. "Moonshine" is home-distilled liquor; "sunny trifles" can be understood as "bright little details". The image here is that you collect the details in the text (as if they were fruit) and after thinking them over ("fermenting" them), your general thoughts on the topic will be a "distillation" of the details. The rest of the paragraph supports this analogy: details first, generalizations later.

I think he means that readers should first care about the story itself and the details (sunny trifles) that the author gives to make the world of the story. Then, it's ok to look at or take in the bigger meaning (the moonshine) of the story (the theme, the message, perhaps). I don't think he means moonshine in the sense of a kind of alcoholic drink.