The color that is seen behind the content. Usually, for background white or some neutral tints are used, for instance #FFFFFF. The main feature of the background color is high contrast with text color, not less than 80%.
If some dark colors are chosen, then, the high-chroma colors yellow (#FFFF00), green (#00FF00), cyan (#00FFFF), and magenta (#FF00FF) suit best. White here is over contrasting, providing high tension of eyes and worse legibility.