This article discusses three options for enabling and disabling Safari's dark mode on your Mac: through System Preferences, through Safari Reader View, and through a browser extension.
How to Enable Safari Dark Mode on Your iPhone (New Feature!)
At night, black text on a white background is particularly hard on your eyes. Enabling Dark Mode for your Mac is incredibly easy. It doesn’t just turn on Dark Mode for Safari, it does it for all your apps, and it’s perfect for protecting your eyes late at night or in low-light conditions.
Click the Apple icon at the top left of the screen.
Click on System Preferences.