The chat window is composed of two parts. The outer part is on the website the visitor is currently on. The inner part is a iframe loaded from the giosg servers. For the outer part you can use any fonts that are already loaded on the page.
If you want to use custom fonts for the elements in the iframe (such as in the chat messages) you need to add the font-face declarations under the iframe template. You might need to allow the font's to be loaded from the service.giosg.com domain. You can also upload the fonts to the giosg assets in your account and use those urls in your font-face declaration.
