XFCE not using standard Mint-Y theme when starting from console

[mgr]

Hi,

I'm using Linux Mint LTS Xfce for a long time and recently I have upgraded from 17.3. to 19.1.
I use this system in a special remote boot environment with XDM als login manager. In some
cases (e.g. for events or conferences) the system directly boots to the desktop without
using the login manager (autologin with startx and dynamicaly created users). Basically
everything works fine, but when the system directly boots to the desktop, xfce do not use
the standard theme "Mint-Y". Instead, a generic oldfashioned one is used. The same happens
if I start X from the console login. This behavior does not exist in version 17.3. Has anyone
an idea what's going wrong and how it can be solved (another login manager with autologin
feature is no option because of other technical requirements)?

Thanks,
Martin