i want to open my website pages in dark mode

Hello,

If you want to open website pages in dark mode, first check if the website has a native dark mode. If not, you can install an extension like Dark reader to turn UI elements dark. Note that some colors may be inverted, and some configuration is required, and may break the color scheme of some websites.

There's a setting for content appearance. And then, your websites would need to support multiple color schemes. See support.mozilla.org/questions/1589835 for more details.