If anyone is having issues with dark mode on Facebook here is a workaround. So sometimes the dark mode option doesn't show up on Facebook for me. Sometime when I get it to turn on sometime it automatically turns off. I noticed on the ZF3 if I open Facebook unfolded dark mode is off, but unfolded dark mode is on. So simply open Facebook unfolded with dark mode and fold open the device and it will switch to the main display in dark mode.