Many forum and webpages have the option to switch to "dark mode." It's sort of an inversion of color. Your background will be a dark color (black or dark grey, for example) while text will be a lighter color (white, yellow, etc.) It's a lot easier on the eyes.
Some websites give you a choice of "themes" that change the way the site looks. Different colors for the background, text, and other things. Kind of a way for you to customize it. Dark theme or night mode is a dark background with white text. Some people prefer it.