Dark mode is a feature that is not incorporated in all web site designs. A traditional website has one design for background and font color choices. A website that has dark mode capability has a second choice of background and font color choices that is less brilliant at night. “Dark mode” just tells the browser which color scheme to use. I have found that maybe 50% of the websites that i use support dark mode.
However, there are some browser extensions (for both chrome and safari) that try to provide a “dark mode” for all websites by forcing different (background and font) color schemes on websites for dark mode. This would work fine for switching between black and white but may look weird for or other colors. Search for “dark mode” in the app store to find an extension.
Chrome also has a built in flag that you can change that tries to do this. But i have not tried this.
https://www.pocket-lint.com/apps/new...google-chrome/