Create a new XML file and add it to the __openerp__.py file as dependency. An example of such a template:

<openerp>
    <data noupdate="1"> 
       <record id="your_template_email" model="mail.template">
            <field name="name">Template title</field>
            <field name="email_from">${object.event_id.user_id.email or ''}</field>
            <field name="subject">${object.event_id.name} - Reminder</field>
            <field name="model_id" ref="calendar.model_calendar_attendee"/>
            <field name="email_to" >${('' if object.partner_id and object.partner_id.email and object.partner_id.email==object.email else object.email|safe)}</field>
            <field name="partner_to">${object.partner_id and object.partner_id.email and object.partner_id.email==object.email and object.partner_id.id or False }</field>
            <field name="auto_delete" eval="True"/>
            <field name="body_html"><![CDATA[
                <style>
                    span.oe_mail_footer_access {
                        display:block;    
                        text-align:center;
                        color:grey;                                
                    }
                </style>
                <div style="border-radius: 2px; max-width: 1200px; height: auto;margin-left: auto;margin-right: auto;background-color:#f9f9f9;">
                    <div style="height: auto;margin-left:12px;margin-top:30px;">
                        <p>ANY TEXT YOU WANT HERE!</p>
                    </div>            
                </div>
                ]]>
                </field>
        </record>
    </data>
</openerp>

As you can see you can add CSS etc in it. An important note is to add data noupdate="1" in it though! So the template is never reloaded.