Steps to reproduce the issue

1. Go to System - Mail Templates
2. Open the Options
3. for the field "Mail Format" choose "HTML" or "both", Save
4. Send an email through Joomla (i.e. forgot password or Test Mail configuration in global configuration)
5. Read mail with MS Outlook

Expected result

Mail body background should be light grey (i.e. in Thunderbird)

Actual result

Mail body background is white in Outlook

System information (as much as possible)

Joomla 5.2.4

Additional comments

The background color is configured with #00000008
But Outlook cannot show transparent colors