The use of fonts is currently limited by support in e-mail clients.
We advise using web-safe fonts to ensure your signatures are displayed the same way in different e-mail clients.
Also, Gmail doesn't support methods for importing or linking fonts from external sources; it strips header styles, and there's no way to inline custom font declarations.
Email has so many more constraints than the web because of the many and different ways the HTML is consumed and altered by clients.
You'll need to inline your CSS and use font stacks that fall back to acceptable system fonts.
How to set up fallback fonts
This is one of the more advanced modifications that actually requires dealing directly with the code of your signature templates.
If you wish to set fallback fonts, go to Source view in the editor and find the part you wish to set.
You can see the current font, just apply your changes to it as shown in this snippet:
<span style="font-family: 'Roboto', Arial, sans-serif;">Text</span>
If you apply this, the first font will be applied; if that won't be possible, the second one. Don't forget to include some generic font family as a 'last resort'.
If you don't have any experience with code, don't worry and contact us, we'll be happy to help you!