I’ve created myself a black background image at the resolution of the phone screen, and I also applied a script from x10ii-x10iii-color-banding-in-low-light-conditions which fools the system and gives me true black colour. Still tho many apps, especially the browser, have bright or white colours which does not help much.
In any case power consumption on the screen is not the main issue, the phone keeps consuming lots of power while idling, in my case it’s about 1% per hour if not more.