Not an admin on a multisite setup but I want to change the fonts

As you can see here, CSS is a security risk. In a multisite the style tag is even stripped from posts and pages unless the super administrator is editing the post/page.

Suggesting that they allow you to create child themes is not good. They will have to give you access to the PHP files, which is even worse.

The best would be for them to install a custom CSS plugin that is limited to only safe rules (if such a plugin exists).

At the moment you can use the style attribute inside your posts and pages to change the font of all the text in the content.