Regarding the themes, I definitely can't do the light-text on dark-back on a monitor, though I'm fine with it on a smaller screen. Tablets and laptops still tend to be large enough that I go for the lighter background, but on phones and such I'm indifferent either way.

Most of that is because I have suuuuuuper bad eyes and I need background lighting when I do anything, especially with movies and such. I can't focus on the bright point in front of me without other sources of light.