diff --git a/prefs.js b/prefs.js
index 1dc41d6..e079cc3 100644
--- a/prefs.js
+++ b/prefs.js
@@ -4,4 +4,7 @@
 //
 
 // PREF: customized ui layout
-user_pref("browser.uiCustomization.state", "{\"placements\":{\"nav-bar\":[\"back-button\",\"forward-button\",\"stop-reload-button\",\"urlbar-container\",\"downloads-button\",\"fxa-toolbar-menu-button\"],\"toolbar-menubar\":[\"menubar-items\"],\"TabsToolbar\":[\"tabbrowser-tabs\",\"new-tab-button\"]},\"currentVersion\":18}");
\ No newline at end of file
+user_pref("browser.uiCustomization.state", "{\"placements\":{\"nav-bar\":[\"back-button\",\"forward-button\",\"stop-reload-button\",\"urlbar-container\",\"downloads-button\",\"fxa-toolbar-menu-button\"],\"toolbar-menubar\":[\"menubar-items\"],\"TabsToolbar\":[\"tabbrowser-tabs\",\"new-tab-button\"]},\"currentVersion\":18}");
+
+// PREF: use dark theme
+user_pref("extensions.activeThemeID", "firefox-compact-dark@mozilla.org");
\ No newline at end of file