For rich web applications with dynamic contents using javascript and ajax, ARIA is used. (https://www.w3.org/WAI/intro/aria) You can see them a lot in the Bootstrap examples.

Showing codes looks like a suitable case, but why?

You can try your sense for kerning at http://type.method.ac/ . It is quite fun :)

It reminds me of traffic lights...

What about code editors, such as Visual Studio or Sublime Texts? Also, terminals usually uses dark backgrounds. I think those are examples opposed to this section. Maybe it is due to programmers normally doing their work at night.. :)

Plus, I think this is a good article on dark backgrounds. http://tubikstudio.com/dark-side-of-ui-benefits-of-dark-background/

Still, it is rather common to use dark background color for image manipulating tools (e.g. Adobe Photoshop & Lightroom), since the visual content itself should be focused and the UI components need to be fade out. It looks like the site is about colors, making it somewhat similar to this case.

Taskbar of Windows XP used colors with high saturation. It was okay back then, but it looks like a mess after 15 years...