If the website doesn't support Dark Mode, you can use Safari Reader to read articles in Dark Mode. When you use Dark Mode, Safari automatically shows a website in Dark Mode if the website has been designed to support it. To use a light background for notes while Dark Mode is turned on, open Notes and choose Notes > Preferences, then deselect “Use dark backgrounds for note content.” To use a light background for maps while Dark Mode is turned on, open Maps and choose Maps > Preferences, then select ”Always use light map appearance.” In earlier versions of macOS, click View in the menu bar in Maps, then deselect Use Dark Map. To use a light background for email messages while Dark Mode is turned on, open Mail and choose Mail > Preferences, then click the Viewing tab and deselect “Use dark backgrounds for messages.”
Some apps and features have special Dark Mode settings or behaviors.