Dark mode uses a darker background with lighter text and controls. It is like reading a sign under dim evening light, with the bright parts kept small and clear.
Some people prefer this look or find it more comfortable in a dark room. A site may let them choose it or follow the color preference set on their device.
Check that text, buttons, and images remain easy to understand in both themes. Dark mode is not simply swapping black for white; colors need to keep enough contrast to be readable.