Hi,

    Go to:

    Settings > Technical > Email > Email Templates

    Select your email template from the list.

    Under the "Body" tab:

        You will see a field called "Body HTML" - this is where you enter the HTML content of your email.

        You can customize this HTML however you need - use headings, bold text, tables, colors, links, etc.

        Leave the "Body" (plaintext) field empty so that Odoo sends only the HTML version.

    Save the template.

Example HTML content:

<h2 style="color:#2E86C1;">Welcome to Our Service!</h2>

<p>Dear <strong>${object.partner_id.name}</strong>,</p>

<p>Thank you for registering. We're excited to have you on board.</p>

<p>Click the link below to get started:</p>

<a href="https://yourwebsite.com/login" style="color:#ffffff; background:#28a745; padding:10px 20px; text-decoration:none; border-radius:5px;">Login Now</a>

<p>Best regards,<br>Your Company Team</p>

This will send a clean, formatted HTML email - without any plain text fallback.

Hope it helps.