Anna,
Thank you for starting the discussion. Put me down for 'Dark' please.
Defining 'light' and 'dark' is not easy. I've lately been thinking of pictures as being 'Decorative', 'Documentary', or 'Expressive'. Of course many photo themes will span two, or even all three of those arbitrary categories.
When making pictures, I try for 'Expressive' when I can, or at least 'Decorative / Somewhat Expressive'.
When viewing other people's photographs, my preference is for 'Expressive', because I crave images that make me think or feel. They are often dark and moody. I think the best of them have a glimmer of light within, though, offering some wistful hope or positive thought that shines through the darkness.
At the bottom line, there are photographs that say something that resonates with the viewer, and those that don't. In that second group, some try to communicate, but miss the mark - while others are just pleasant to look at.
Steven