How do I call a javascript function from Google Chrome Dev console?

When I change the theme of Home Assistant (by clicking radio button at user’s profile, demo), it calls fireEvent, if I read the source correctly.

I would like to call it from the browser’s console, but if I do

fireEvent(this, "settheme", { True });

I get

VM1775:1 Uncaught ReferenceError: fireEvent is not defined
    at <anonymous>:1:1

How can I access this function? Later I will call this function from the TamperMonkey script.