Trying to follow the idea of customizing fonts in our Confluence cloud instance has lead me to this page, on Theme creation: https://developer.atlassian.com/cloud/confluence/theme/
It says at the top: The Theme API was built for a UI that Confluence has recently moved away from, and doesn't support the new UI. Based on Theme API usage to date, we are evaluating whether Theme API will support the new UI.
So, Atlassian, what's the status ? Have you made up your mind ?
I sure hope, there will be some way for cloud customers to actually customize our own pages. I am really surprised to find out that a simple look and feel change, like changing font, is not a core feature! Read any article on how to make your company texts stand out and you will read about visual identity, including fonts and colours.