Hi Sven,

Only safe HTML is allowed in the "Custom footer" setting for security reasons, so the style tag will be getting stripped.

In order to add custom CSS you'll need to add the styles to the "Custom CSS" setting in the site-level theme settings. If you want to target a specific tenant only, there are tenant specific classes added to the body tag that you can use, e.g.

.tenant-user-mytenantidentifier .breadcrumb-nav { … }

Kind regards