If you are an admin user, you can change the theme of a project. To do so click Menuin the top right --> click Theme
Advanced: CSS customization
If you use code notation on your profile page, and create a code block starting with code:style.css and add CSS in it, you can create custom CSS for that user when they use that project.
If you create a page called 'Settings' in your project, and add a code block: 'code: style.css' and add CSS in it, you can apply styling across that entire project.