into the file .iconrc where you define the stock icons used by GTK2-XFCE42 new
themes, some icons definitions are bad (or better, are different respect the
icon name)

in .iconrc I found this :

stock["gtk-color-picker"] = {{ "stock_color_picker.png" }}
stock["gtk-dialog-error"] = {{ "stock_dialog_error.png" }}
stock["gtk-dialog-info"] = {{ "stock_dialog_info.png" }}
stock["gtk-dialog-question"] = {{ "stock_dialog_question.png" }}
stock["gtk-dialog-warning"] = {{ "stock_dialog_warning.png" }}

but the name of icons file installed is 

stock_color-picker.png
stock_dialog-error.png
stock_dialog-info.png
stock_dialog-question.png
stock_dialog-warning.png


My system : slack9 with xfc42rc1 installed from graphic installation wizard (