Would using black background and light foreground reduce the LCD power consumption? Or actually increases it? Please note, it is not only the power consumption used for luminocity of LCD that needs to be taken to consideration but also the power used by the LCD to switch off and on the cells. Of course this power consuption is geared towards reading text documents than playing videos or proccessor intensive tasks.
If the panel is an OLED, yes that would reduce power consumption considerably. If it's a TFT panel, displaying black/dark colors would not reduce power consumption -- source:
If you have a TFT panel, the only way to reduce the display's power consumption is to reduce backlight brightness, contrast and perhaps turn off optimizations on the display (if any).
In general a current is going to be applied whether it is black or white. This is because to produce black, it has to send a current to align the crystals. Since there is no "black" subpixel, it has to send a current through all of them. The same with white. In general, there is going to be a red, green and blue subpixel. Some LCDs have one red, two green and two blue, cause those colors are harder to produce vibrantly.
But that's just theory. Practically, there is more to power consumption that subpixel. More importantly are factors like the efficiency of the power inverter, battery, etc etc. Turning off power to a "cell" isn't like shutting off a gate and the electrons just stand still. Rather, the electrons are usually bled off in heat. So the power is used one way or another in most cases.
This is why I can safely say you will see no difference in power consumption between different color backgrounds.
You are overthinking this man. Your priorities should be else where. Want to save energy? Learn to speed read. Increase your typing speed. Get Autokey and define text snippets. Play a game of memory to sharpen your mental skills. These are the skills that will save you tons of energy.