In Axigen X there are only limited branding possibilities.

In earlier versions there were more options here - siehe https://www.axigen.com/articles/advanced-webmail-skinning-sass-axigen-x3-update-2_76.html

It would be great, if would be possible again to create custom themes based on domain scope.

If the effort involved is too great, then it would help, if a custom css could be entered in domain configuration so css classes can be overwritten