Hi,
You can change the font by specifying the font-family, but keep in mind that the list of web-safe fonts are not so long. If you want to go crazy with the font you can use Google Fonts by adding the CSS file reference over the Custom CSS field and then referring the font name in the Field CSS as AdamP suggests, or you can target all field labels by adding to the Custom CSS textarea.
For example add this to the textarea above Custom CSS:
https://fonts.googleapis.com/css?family=Notable
and then add this to the Custom CSS to change all fonts:
.ms-standardheader{
font-family:"Notable";
font-size:14px;
}
Use this (or add it to the Field CSS to change only one:
#dffs_YOUR_FIELD_NAME .ms-standardheader{
font-family:"Notable";
font-size:14px;
}
Change YOUR_FIELD_NAME withe the name of the field you want to style.
Hope this gets you started.
Best regards,
Alexander