does using darkmode on fp3 save energy?
At 20% battery energy the phone turnes into battery saving mode which includes darkmode.
But as far as I can remember darkmode doesn’t save energy on fp3 due to the screen technology. Is that true? I tried to verfy that on the forum but I couldn’t find an answer.
The FP3 has an IPS display panel and therefore doesn’t illuminate individual pixel, which means having more black pixels won’t save energy. So you’re correct! 
Many thanks, @Switch! 
So I try to change the energy saving settings to prevent the darkmode which I really dislike.
Yes, the pixels are not lit individually. Even the “black” ones are lit all the time, they just don’t let the light through.
Dark theme saves energy with other display technologies such as OLED.
Unfortunately I don’t think that’s possible. I notice that in Battery Saver mode the Dark theme is locked (On). If you find a way, let us know 
Well, @OldRoutard finally I was a bit too enthusiastic believing I could disable dark mode in battery saver mode. Indeed I failed. Does anybody know a way without root?
Paradoxically, on IPS displays, dark mode usually drains more energy. From my personal experience, you need higher brightness levels with dark mode when you’re e.g. outdoors than with light mode.
Energy saving mode usually disables the performance cores (hence the device feels slower) and also reduces the brightness.
Dark mode only saves energy if you use OLED and if the dark mode is completely black so that the pixels are disabled. On IPS, dark mode is dark gray. Some apps do have settings for OLED to use black (disabled pixels) instead of dark gray (enabled pixels).
I think this app can do that, unfortunately it needs root-privileges but I will leave the link here anyways in case someone who stumbles across this thread can make use of it.