Staring at a bright light makes floaters visible, so dark is a must for me. Plus, code should never be dark-on-light, that's disgusting.

Astigmatism can cause eye strain headaches. I don't know if LASIK corrects that or not, or if you have it, but getting glasses with cylindrical correction helped reduce some of my headaches. Apparently most people don't see a big starburst around lights at night, or have a faint/fuzzy halo around text at any distance.