I am using Adwaita-mörk.

>Settings>Appearance offers 2 more choices : Adwaita HighContrast

/usr/share/themes/ contaisquite a number of themes among which
Adwaita Adwaita-dark HighContrast Default 'Equinox Evolution Dawn Squared' Industrial Retro Bright Default-xhdpi Kokodi Smoke Clearlooks Emac Mist ThinIce Crux Moheli 'ZOMG-PONIES!'

when I try to drag then into the Appearance window the 3 items which are there disappear and window stays blank. When I close it and activate
>Settings>Appearance again the 3 original items appear but not the one I did drag in.

I have googled an awful lot and have not being able to find information which helps me with this. Do you have any tips ?

I think on XFCE you have to use XFCE-settings to change the theme. No drag-n-dropping involved.

@ ondoho

much late thanks ondoho !