It is the reason a reader should want to hear the story you have to tell. In short, theme is the new insight about life or human nature that you hope to reveal to your readers. Let the result of the conflict reveal the theme. How characters react to the conflict can show the thematic insight. Your story should be a specific illustration of the theme.
Personality traits are more important to show than physical descriptions.
Foreshadowing should build suspense by keeping the readers guessing. Give subtle clues, not clear statements. Sometimes, an event could really take place anywhere. In this case, scene is of less importance. Skip the details and give only an overall impression of the setting.