This is one way to hard code your Fancy Fonts in Signatures, Letterheads and Footers, as oppose to writing HTML.
In regards to setting up a signature with particular fonts in HTML.
Font choices have and are always limited in compatibility and availability from sender to recipient.
What the creator of the email template (sender) might choose, might not necessarily be available for the recipient of the email (either not stored on their local machine or server). This will result in font defaulting to something similar but not necessarily a like and throw out all your desired look and feel.
Another way around this is to host these fonts of choice on a server and refer to them through Style sheets, but this too is an issue as the User needs experience and expertise in writing HTML and style code.
The easiest and perhaps the most common way to make sure of a font is keeping its desired look and feel is to include it as an image in a Letterhead (as a Header, Footer, Signature).
Design a letterhead with only a 'header footer or signature' and attach this to the email that you create.
Using a stored image or editing program/application create an Image of the Letterhead or Footer save it in your personal or public Documents folder (Documents Tab in Salesforce CRM / Your Org).
NOTE: Try to keep the image width to 600 pixels maximum, height is variable. Jpeg or Gif Image only. Optimize to 100dpi or less (web default is 72dpi).
Go to Set up / Admin Setup/Communication Templates / Letterheads
Create a New Letterhead (Footer).
Design the Letterhead from the Default template displayed.
NOTE: for a total white letter you can just change the color to #FFFFFF (white) and enter a pixel value of '0' in height for each of the elements.
Select image via the 'Select Logo' button and click OK once you have located it. If you do not see it in your documents folder, it is because you might have not made it accessible.
NOTE: Be sure to make it accessible externally so you can select it when assembling your Letterhead.
Once done and you have previewed the Letterhead, click Save.
Now as done previously go to Set up / Personal Setup / Email/My Templates.
You can now build your email Document and select your NEW Letterhead/Footer.
Save and Send as usual.