diff --git a/src/userChrome.scss b/src/userChrome.scss
index 31ddff1..4f87589 100644
--- a/src/userChrome.scss
+++ b/src/userChrome.scss
@@ -16,7 +16,9 @@
 }
 
 /** System Default Theme ******************************************************/
-@import "theme/system_default_theme";
+@include Option($systemDefault) {
+  @import "theme/system_default_theme";
+}
 
 /** Fully Theme Mode **********************************************************/
 /* Default Themes
diff --git a/src/utils/_option.scss b/src/utils/_option.scss
index 1ed3a53..2544de8 100644
--- a/src/utils/_option.scss
+++ b/src/utils/_option.scss
@@ -1,9 +1,10 @@
 $tabProton: "userChrome.tab.proton";
 
-$builtInContrast: "userChrome.theme.builtInContrast";
+$builtInContrast: "userChrome.theme.built_in_contrast";
 $compatibility: "userChrome.theme.compatibility";
 $compatibilityTheme: "userChrome.theme.compatibility.theme";
 $compatibilityOS: "userChrome.theme.compatibility.os";
+$systemDefault: "userChrome.theme.system_default";
 
 @mixin Option($optionName) {
   @supports -moz-bool-pref("#{$optionName}") {
diff --git a/user.js b/user.js
index aca2006..bec8442 100644
--- a/user.js
+++ b/user.js
@@ -36,10 +36,11 @@ user_pref("userChrome.panel.proton", true); // Original, Proton
 // user_pref("userChrome.panel.photon", true); // Photon
 
 // == Theme Default Settings ===================================================
-user_pref("userChrome.theme.builtInContrast", true);
+user_pref("userChrome.theme.built_in_contrast", true);
 user_pref("userChrome.theme.compatibility", true);
 user_pref("userChrome.theme.compatibility.theme", true);
 user_pref("userChrome.theme.compatibility.os", true);
+user_pref("userChrome.theme.system_default", true);
 
 // ** Useful Options ***********************************************************
 // Integrated calculator at urlbar