To change the embedded font, find in the page head link tags.
<link href='http://fonts.googleapis.com/css?family=Fira+Sans:300,400,500,700&anp;subset=latin,cyrillic-ext,cyrillic' rel='stylesheet' type='text/css'>
To change the font, first go to Google Fonts, choose a font and replace the code above with the generated code.
Next find the _theme.less file in the css/less directory.
At the begining of the document find font variables and change it's content with your new font name
@font1: 'Fira Sans', sans-serif; @font3: 'Droid Serif', serif;