Dark Mode seems to be going mainstream this year... Somewhat. iOS and Android have added it and it seems that some of the major social networks (at the very least Twitter) have as-well. With one flip of a switch, everything can go dark on a phone, along with any websites that are "dark mode compatible" (there is a media query for it, so you can style it in the CSS, I believe). Discourse has a dark mode of sorts, as does Gosora (by sneaking both light and dark themes into the default distribution), but these have to be toggled manually, if I recall. This is a very interesting development and it comes about to make sites easier on people's eyes, particularly in situations with poor visibility or at night.