how do i turn on dark mode on safari

Safari should respond to whatever you have set for the system appearance in System Settings ➜ Appearance

If you have the system set to dark mode, Safari will display its menus and buttons in dark mode.

Websites, will have to support it themselves and usually specific option or button somewhere within them to enable a dark mode. But that is entirely down to the website. Most do not.

There are extensions for Safari you can get that force Dark Mode in websites, but as they change coloring and fonts it may not work 100% with all websites.

https://apps.apple.com/us/app/nightshift-dark-mode/id1561604170?mt=12

Look for Safari extensions that produce a dark effect on web pages such as these communities which otherwise knock your eyes out with white backgrounds. In Safari menu > Extensions… Search for these titles:

• Dark Reader for Safari
• Noir - Dark Mode for Safari

Click on the blue cloud download symbol to install your choice into Safari Settings > Extensions.

I use Noir in Safari 18.5 and Dark Reader as a Firefox 140.0.4 extension.