Thanks @Sal!
I figured it out!
First the permissions on the custom.css file were wrong, then I verified also the public/assets directory and found that all *.css files had the wrong permission.
This is how I fixed it:
systemctl stop zammad
vi /opt/zammad/app/assets/stylesheets/custom/custom.css
chmod 644 /opt/zammad/app/assets/stylesheets/custom/custom.css #rw-r-r
zammad run rake assets:precompile
chmod 664 -R /opt/zammad/public/assets/*.css #rw-rw-r
systemctl start zammad
Now the CSS is correctly read and applied!
In case anyone wonders, this is my CSS to hide the sidebard (Activity Stream) and the First Steps in the Dashboard area:
.tab[data-area="first-steps-widgets"],
.sidebar.optional {
display: none;
}
Thanks a lot for your help and suggestions
P.S.
Nice easter-egg 
