The URL link in the first text field is what is calling the font via google. On Apex it is "Muli" I think it was. Basically you need to go to Google Fonts and choose a font you would like, input the URL to the font just like it is currently.
Then navigate to Style Properties -> General -> Body -> Font
The body font controls nearly all font except in messages. Once there you will see 'Muli' , just replace with the name of the font you referenced in the URL. If you need some help or better explaination just let us know.