Reading vs. Scanning
When it comes to text on websites, users either read or scan. Reading involves focusing on words for a thorough comprehension of the subject. Scanning involves skimming the words for a broader comprehension of the subject. When you should use white text on a dark background depends on whether users are scanning or reading text.
The kind of text that users read is paragraph text. You should avoid using white text on a dark background when displaying paragraph text to make it easier from them to read. Forcing users to fixate on the white text for a long time can strain the user’s eyes. This is because white stimulates all three types of color sensitive visual receptors in the human eye in nearly equal amounts [source]. This makes reading white paragraph text on dark backgrounds stressful on the user’s eyes.
This guy says the opposite
The high contrast themes change the background to black and the text to white. This high contrast theme is vastly easier on the eyes and reduces eye strain. If you’re looking at a monitor for extended periods, this will make your day easier. It will seem weird at first, but give it a try! You will wish you had found this sooner!