Hi PCK WORLD,
You need to contact support. As the Creator FAQ mentions:
Can I add custom CSS for my game page?
You sure can… but it’s disabled by default. First make sure what you are trying to design can’t be done with the built-in theme editor. If it can’t, then contact us and we’ll get you set up with the CSS editor. It’s disabled by default to ensure a certain level of consistency on project pages to make it possible to add new features without breaking things.