To edit the template of a certificate, again from Account and Settings move to the Certifications option (2).
This is also available when editing a unit and selecting code view (3), allowing you to modify the unit's content in html.
The editor has syntax highlighting that greatly improves the readability of the code but there are also some advanced features that you might find useful:
Ctrl-F / Cmd-F
Start searching (4)
Ctrl-G / Cmd-G
Shift-Ctrl-G / Shift-Cmd-G
Shift-Ctrl-F / Cmd-Option-F
Shift-Ctrl-R / Shift-Cmd-Option-F
Persistent search (dialog doesn't autoclose, enter to find next, Shift-Enter to find previous)
Jump to line
Regular expressions can be used in searches
Full screen mode
You can press F11 when the cursor is in the editor to toggle full screen editing (5).
To exit the full screen you can press Escape button.