Dark mode can cause strain on eyes. Lower total brightness means your pupils open up to be able to take in more light. Wider pupils means lower depth of field,

Dark mode can cause strain on eyes. Lower total brightness means your pupils open up to be able to take in more light. Wider pupils means lower depth of field, i.e. less stuff in focus immediately in front of/behind whatever you are currently focused on. You are more susceptible to having to refocus from subtle movements with a lower depth of field. The less of a steady state your focus, the more the muscles involved in focusing are engaged, which might be a cause of the strain you are feeling. Similarly, this is why your eyes can hurt while reading in dimmer settings. It's not because low light itself is somehow harmful while reading, but because your eyes have a harder time staying focused as you naturally move while reading. edit: I adopted light themes everywhere after reading this, probably https://ux.stackexchange.com/a/53268 https://ux.stackexchange.com/a/53268 . Knowing depth of field from photography helped this make sense, as well as understanding (from conversations with a doctor in my family) that the generic feeling "strain" is most likely associated with muscle activity than anything else.