In addition to offering a WYSIWYG (What You See Is What You Get) editor to edit your e-mail templates, you also have the ability to edit the HTML (to get even fancier).
Head to your Account Settings (the gear icon in the upper right).
Next, select Templates and then Email Templates from the sidebar on the left.
Find the template you would like to modify and click the Edit button under the Actions column, then the Edit HTML button:
This will change the text in the viewer from the WYSIWYG style to the HTML code. Please note that we are unable to directly support any custom HTML coding, so please make sure to attempt this only if you are comfortable with HTML.