hii timochan86,

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

[https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save]

Thanks Regards, -Arman

You can change this pref on the about:config page from 3 to 2 to revert to the Firefox 94 behavior.

• layout.css.prefers-color-scheme.content-override
  Dark (0), light (1), system (2) or browser (3)

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

See also this pref:

• ui.systemUsesDarkTheme [0:light; 1:dark; 2:no-preference]