Easier on the eyes surely, but I understood it that dark themes only save battery for OLED/AMOLED screens. For LCD/TN dark themes use slightly more. I am not an Electrical Engineer though.
Which makes AMOLED still not old enough to enter the general consciousness of the designers. They're optimizing for the screens they (and everyone they know) are used to.
Up until very recently, OLED were great if you didn't care about color reproduction. My 2011 Nokia N9 had an OLED display. Then LCD made something of a comeback due to superior color fidelity. Currently OLEDs are getting good enough to perhaps finally replace LCDs for good, although there are some promising unorganic LED display technologies already around the corner...
