5000s: add ui.systemUsesDarkTheme

user.js

@@ -1616,6 +1616,7 @@ user_pref("_user.js.parrot", "5000 syntax error: this is an ex-parrot!");
    // user_pref("browser.download.autohideButton", false); // [FF57+]
    // user_pref("toolkit.cosmeticAnimations.enabled", false); // [FF55+]
    // user_pref("toolkit.legacyUserProfileCustomizations.stylesheets", true); // [FF68+] allow userChrome/userContent
+   // user_pref("ui.systemUsesDarkTheme", 1); // [[FF67+] override OS: 0=light, 1=dark
 /* CONTENT BEHAVIOR ***/
    // user_pref("accessibility.typeaheadfind", true); // enable "Find As You Type"
    // user_pref("clipboard.autocopy", false); // disable autocopy default [LINUX]