Light mode all day and night long.
I really feel that dark mode is much more tiring for my eyes, compared to light mode.
There's something about white letters on dark backround, that makes it almost impossible for me to read long texts without feeling uncomfortable. The letters kinda appear blurrier compared to black on white.
I also remember reading a scientific article that explained why it's not good for the retinal to keep constantly reading white text on dark backround (can't find it right now, it was a long time ago when I searched about this, I really wanted to persuade myself to permanently use dark mode).
Since iOS enabled this feature, I have tried multiple times to get used to it and the most I've managed to use it is ~10 days.
I always feel quite relieved after re-enabling light mode.