Im wondering if the one button thing is a theme or a restriction?

Yes, to me one button is the theme, but it also a restriction.  I take it as the gameplay should be centered around one button. With that said, now you can use more than one button, if it does not affect the gameplay in any fashion, see the link below from the Game Jam's host. I have utilized this for navigation to different scenes (Main Menu, Information, Game itself, end game scene, and the online leader board) for example. No additional buttons have an effect on the core gameplay itself, and it would be too tedious to the player to have to memorize keyboard shortcuts to navigate the game menus.

https://itch.io/jam/boundless-game-jam/topic/1482829/can-i-use-an-extra-button-j...