Finally a have found a solution. The decisive hint came from DolphinOracle (MX Linux):
the xfconf backend for Xfce does not run for both root acounts and the user account, it only runs for the account that is logged in
So root must be kept from reading the user-theme and icons.
Therefore I have created 2 folders in /usr/share and called them
/usr/share/icons_home and /usr/share/themes_home.
To these folders I have moved my user icons and theme (Numix). Then I have created links from my home directory to these folders.
Now root cannot read my user theme/icons anymore and is using the default xfce4-theme and gnome icons instead.
So I have a light theme for root and a dark them as user. Their difference is quite obvious now.