Give this a try, never editied email notifications before, ive just left them as is, BUT I did find this READ ME in the templates folder
'cat /opt/observium/includes/templates/README'
Here place for default templates for alerts, groups and notifications.
DO NOT TOUCH files here!
Use user template overrides in main observium dir templates/
For example:
# change current dirrectory to your observium dir
cd /opt/observium
# copy (or create) new template from defaults, for example HTML email notifications:
cp includes/templates/notification/email_html.tpl templates/notification/
# edit copied file:
nano templates/notification/email_html.tpl