diff --git a/ext/bg/js/settings/popup-preview-frame.js b/ext/bg/js/settings/popup-preview-frame.js
index 8fd06222..a7863b3a 100644
--- a/ext/bg/js/settings/popup-preview-frame.js
+++ b/ext/bg/js/settings/popup-preview-frame.js
@@ -137,7 +137,7 @@ class SettingsPopupPreview {
 
     setCustomOuterCss(css) {
         if (this.frontend === null) { return; }
-        this.frontend.popup.setCustomOuterCss(css, true);
+        this.frontend.popup.setCustomOuterCss(css, false);
     }
 
     async updateSearch() {
diff --git a/ext/fg/js/popup.js b/ext/fg/js/popup.js
index 970c5343..6fbab62b 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -158,11 +158,11 @@ class Popup {
         this._container.dataset.yomichanSiteColor = this._getSiteColor();
     }
 
-    async setCustomOuterCss(css, injectDirectly) {
+    async setCustomOuterCss(css, useWebExtensionApi) {
         // Cannot repeatedly inject stylesheets using web extension APIs since there is no way to remove them.
         if (this._stylesheetInjectedViaApi) { return; }
 
-        if (injectDirectly || Popup._isOnExtensionPage()) {
+        if (!useWebExtensionApi || Popup._isOnExtensionPage()) {
             Popup.injectOuterStylesheet(css);
         } else {
             if (!css) { return; }