I want to do a Repetier-Server.deb with my own custom. I read
but I don't know how to do the .deb.
1. Check in debian/ubuntu how to create deb files. It is quite easy if you follow the descriptions. After all it is a directory structure with some config files and a call to the packager then.
2. Build the deb to install your extra files and add the original server deb file as well. In post instructions do the installation of the regular server. Since your files are already installed the started server will then show your branding.
3. Don't forget to set the location to look up your branded deb files instead of our server update info. That way you decide when updates are available and users get them including your adjusted brandings with each update.