Is There a Tool for True Night Mode on White Websites

Yes. Windows and macOS dark modes only change websites that support the prefers-color-scheme: dark setting; sites without a dark theme stay white.

Modern Dark Mode handles the rest: it puts a color-flipping filter on any site, however old or plain, turning white backgrounds dark charcoal and text light gray while keeping images normal. For pure black, raise the Contrast slider in Advanced Settings to about 125%; for evenings only, its schedule can turn it on automatically.