diff --git a/ext/bg/js/settings/popup-preview-frame.js b/ext/bg/js/settings/popup-preview-frame.js
index a7863b3a..8c354bf7 100644
--- a/ext/bg/js/settings/popup-preview-frame.js
+++ b/ext/bg/js/settings/popup-preview-frame.js
@@ -22,7 +22,8 @@ class SettingsPopupPreview {
     constructor() {
         this.frontend = null;
         this.apiOptionsGetOld = apiOptionsGet;
-        this.popupInjectOuterStylesheetOld = Popup.injectOuterStylesheet;
+        this.popup = null;
+        this.popupSetCustomOuterCssOld = null;
         this.popupShown = false;
         this.themeChangeTimeout = null;
         this.textSource = null;
@@ -50,19 +51,19 @@ class SettingsPopupPreview {
         const popupHost = new PopupProxyHost();
         await popupHost.prepare();
 
-        const popup = popupHost.getOrCreatePopup();
-        popup.setChildrenSupported(false);
+        this.popup = popupHost.getOrCreatePopup();
+        this.popup.setChildrenSupported(false);
 
-        this.frontend = new Frontend(popup);
+        this.popupSetCustomOuterCssOld = this.popup.setCustomOuterCss;
+        this.popup.setCustomOuterCss = (...args) => this.popupSetCustomOuterCss(...args);
+
+        this.frontend = new Frontend(this.popup);
 
         this.frontend.setEnabled = function () {};
         this.frontend.searchClear = function () {};
 
         await this.frontend.prepare();
 
-        // Overwrite popup
-        Popup.injectOuterStylesheet = (...args) => this.popupInjectOuterStylesheet(...args);
-
         // Update search
         this.updateSearch();
     }
@@ -83,9 +84,9 @@ class SettingsPopupPreview {
         return options;
     }
 
-    popupInjectOuterStylesheet(...args) {
+    async popupSetCustomOuterCss(...args) {
         // This simulates the stylesheet priorities when injecting using the web extension API.
-        const result = this.popupInjectOuterStylesheetOld(...args);
+        const result = await this.popupSetCustomOuterCssOld.call(this.popup, ...args);
 
         const outerStylesheet = Popup.outerStylesheet;
         const node = document.querySelector('#client-css');