Can I override this Javascript CSS myself?

[simonmlewis]

We are using the Zendesk chat service, and the font use is Verdana.
I want to override this to the font we have on our website, so it is more branded.

They state this cannot be done, and you have to pay to do it, but I though that being CSS, I could just override.

If I post the URL here, can someone please take a look to see if it can in fact be overriden? I've tried hard. Just cannot get it to change.