In short:

1. Create a test item holding text.
2. Create a trigger to do a diff() on the text in the test item.
3. Connect your action to the trigger.
4. Use zabbix_sender from the Zabbix server (or another system) to set values.

For details: Ryan Armstrong's Blog - Testing Zabbix actions

By default Zabbix sends plaintext e-mails and has no builtin options to send HTML mail.
See: ZBXNEXT-517 - Mail text as plain/HTML, only on demand

To be able te send HTML e-mail an external script is needed.
Several options exist:

• Viamage - HTML E-mails from Zabbix with sendEmail and a shell script.
• IT Заметки - Отправлять оповещения с Zabbix письмами в формате HTML with mutt and a shell script. (Obviously not written in English, but the scripts and screenshots in de post are in English.)
• JPCorp – Jhones Petter - Zabbix – Media Types Email-Html “sendhtml.py” with libesmtp and a Python script.
• GitHub - dimuskin/ax-zabbix-html-template

1. Go to Administration.
2. Go to General.
3. Set the Filter to “Images”.
4. Click on “Create Image”.
5. Provide a name for the image.
6. Set the type to “Background”.
7. Browse to the file to use.
8. Click on Save.

Source: Zabbix Forums - No Background images MAP - zabbix 1.8.2