Board Thread:Questions and Answers/@comment-25347954-20161002220325/@comment-1892820-20161003005708

I'm pretty sure that Themes let you influence the types of user-generated content that show up in your game. I believe that you can choose from your own creations or any Sporecast that you're subscribed to.

If you choose a theme for your save file, then creations for that theme will be more likely to appear in your game wherever Spore uses pollinated content (e.g. other creatures in Creature Stage, Tribal Stage, or Space Stage; vehicles and spaceships in Civ Stage and Space Stage).