CSS Rules in Theme Editor Not Showing Up

I’m making a fake charity website for a class, but I can’t get the rules I write in the theme editor to apply to my site. Because I’m editing a custom theme given to me by the teacher for the class I’m unable to use the additional css tool, as well as various other backend site editing tools. Any idea as to why this might be?

The page I need help with: [log in to see the link]