dark GTK theme: user suggestion

I added the following line to my ~/.profile

export GTK_THEME=Adwaita:dark

With this line, Caja has a dark side bar.

After choosing the theme Trisquel-dark with the mate-appearance-properties, Caja still has the light side bar on the left. The original side bar is shown here: https://trisquel.info/files/files3_0.png But using the line, mentioned above, let it turn dark.