You mean a themed list? Just go to a story page and at the bottom, there's a category box. Go ahead and click "add category" and type the name of the theme you want to add. It should pop up automatically. Select it and hit "save" and that's it! You can also add themes to a fic while you're creating or editing a fic page. That'll be in the "categories" thing in the right sidebar.

Thank you for asking! I'll add this information to the "adding fics" page!