Even if dark mode "feels" better that still doesn't imply it actually causes less eye strain. Without data it's possible OP's choice could be driven just by aesthetic preferences and they don't realize it.
I found where blue light causes eye strain, and it most certainly does, I immediately notice the difference when using a blue light filter on my monitor, but you can have both dark mode and a blue light filter enabled.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6020759/
Anecdotally, dark mode causes my eyes and my partner's eyes less strain.
You can notice by how dry and red they get, as well as the headaches, it's not rocket science.
EDIT (reached my post limit)
@shawnz: No, the light physically hurts. It's immediate. It's not because of the aesthetics.
I don't think this is "rocket science" but that doesn't mean it is handwavingly obvious either
Having said that I think my preferences are mainly, if not entirely, aesthetic. Dark-on-light terminals feel weird as do light-on-dark documents/spreadsheets - that can't be anything other than just what I'm used to.