Well look at you with your well-lit room.

Someone has to develop and maintain it. It's difficult to quantify the development/datacenter efforts with respect to end user displays. I do doubt dark mode comes ahead at all at any rate. Morealso, OLED displays are overall ill-suited for regular work due to burn-in -- those are the only ones that 'can' save energy (unless you count CRT - but they would be way worse to begin with). The cost of the shorter lifespan OLED (burn-in) would heavily outweight the energy saved - you are looking at 15-20Wh top.

Note: displays with backlight (the extreme vast majority) display blacks by blocking the light, so the dark mode alone doesn't save anything. There are ones with local dimming that can partially benefit.