Battery drain isn't affected on an IPS panel (much) irrespective of the colors in question. I say much because in this setup the backlight is always active, and you need a marginal amount of additional power (~.4%) to strain the crystals to block the backlight.Would I like a dark mode? Yes. Do I think we are going to get a system wide dark mode? No.
Now, if we had an OLED display, I would say more likely. Battery drain with darker colors on OLED is less. Battery drain with lighter colors on IPS is less. So until the display changes from IPS to OLED, I don't think there will be a dark mode.