I think I may have a problem with something naturally missing from the XFCE desktop window management system.
I have installed 3.12.10 of HPLIP on Slackware 13.37.
When I run soome hp-xxxx commands I get a response including:
"QGtkStyle was unable to detect the current GTK+ theme.".

An example is hp-toolbox:
warning: Reportlab not installed. Fax coverpages disabled.
warning: Please install version 2.0+ of Reportlab for coverpage support.
QGtkStyle was unable to detect the current GTK+ theme.
Qt: Session management error: Authentication Rejected, reason : None of the authentication protocols specified are supported and host-based authentication failed
\error: dBus initialization error. Exiting.

Along with the Qt message this seems to indicate perhaps XFCE is not using GTK+ themes (XFCE is fairly bland).

Can someone validate this for me? Or point me in the right direction (disregard the Reportlab stuff I think)?

What theme do you have selected in Appearance Settings?

Does it have a corresponding ~/.themes/<theme-mame>/gtk-2.0/gtkrc file?

Just have "show images on buttons and in menus.
I have no ./themes directory at all. That is part of the confusion here. I have a personal dislike for themes, fonts and cutesy stuff so have no idea why it is citing this.