Where can I find and edit the helpdesk_styles.ftl

Hi Marco. Unfortunately, there is no way to customize this file at the moment. However, you may remove the references to this file from the notification templates and replace it with your own CSS styling.

OK, no problem.

Can you send me the file, so that I know, which styles I have to change?

This file is not accessible, so we can't share it. You may change the relevant notification templates by removing this line:

<#import "helpdesk_styles.ftl" as styles>

As well as all the styles references in the template. Once you've done that, you should have a clean template HTML that you can apply your own styles to.