First, decide on your theme. It could be love, friendship, or something more complex like the struggle between good and evil. Then, create characters that can represent or be affected by that theme. For example, if your theme is jealousy, you can have a character who is overly jealous of their friend's success. Next, develop a plot that revolves around the theme. Maybe the jealous character tries to undermine their friend's achievements. Finally, use the setting and details to enhance the theme. A gloomy setting can emphasize a negative theme like despair.
To write a theme short story, start by brainstorming ideas for themes that interest you. Once you've chosen a theme, think about how you can show it rather than tell it. For instance, if your theme is perseverance, you could write about a character who is trying to climb a difficult mountain. Show their struggles, their moments of doubt, and how they keep going. You also need to have a clear beginning, middle, and end. In the beginning, introduce the theme and the characters. In the middle, build the conflict related to the theme. And at the end, resolve the conflict in a way that ties back to the theme.