Dark themes are easier on the eyes, but light ones can be way more lively if well made. I've preferred the former for quite a long time, but I've recently started to notice how much more memorable the latter have been for me. It must also be the fact that the range of colors you get with light themes is broader than dark themes'; plus, getting access to lighter shades makes for nicer contrast.
Still...light themes are easy to mess up. Good lord, they just pierce through my retinas.