I was wondering if there is a theme involved or we are free to create anything...

As long as you keep to the rules of the comp, including the content rules (kid-friendly),  my understanding was it was up to the authors.

That was my impression, too. The major "theme" is that it must have a tutorial that helps new players to become familiar with text adventures. The plot, goal, storyline, characters, map and puzzles are entirely up to the author.

There is no theme - but perhaps forum lurkers can come up with some tropes that might help a beginner to the genre with ideas for getting started?