Hello S.

Roundcube uses some standard fonts (see config/main.inc.php)

// Default font for composed HTML message.
// Supported values: Andale Mono, Arial, Arial Black, Book Antiqua, Courier New,
// Georgia, Helvetica, Impact, Tahoma, Terminal, Times New Roman, Trebuchet MS, Verdana
$rcmail_config['default_font'] = '';

But, Roundcube uses TinyMCE editor ( program/js/tiny_mce/ )
And you can extend TinyMCE with custom fonts.

How to add Different fonts in TinyMCE Editor
http://kvcodes.com/2013/10/how-to-add-different-fonts-in-tinymce-editor/

Adding new fonts to TinyMCE font options
http://stackoverflow.com/questions/14170758/adding-new-fonts-to-tinymce-font-options

Maybe this is the hint for you. 

Greetings
Markus Heinze