Using custom fonts in Signature 365
If you want to use custom fonts in Signature 365, you can add them to an individual template in the source editor by using code from an external online font repository.
<span> tag. Custom fonts are also typically not supported on mobile devices.Log on to your Signature 365 admin portal, select Signatures and open the signature that you wish to use a custom font with.
Select the Source editor tab to view the HTML source for the template.

You will be presented with the HTML source for the template, the following example will insert the Poppins google font to use in the template. The same process can be used with an appropriate link for other fonts and repositories.
<head>
<link href="https://fonts.googleapis.com/css?family=Poppins" rel="stylesheet">
</head>
Once this is added you can apply this in the signature by using a <span> tag to apply this to the text as required.
<span style="font-family: Poppins, Calibri;"> Your Text Here </span>