diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index d1a34f82..458ea483 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -499,19 +499,30 @@ class Backend {
         return Promise.resolve({frameId});
     }
 
-    _onApiInjectStylesheet({css}, sender) {
+    _onApiInjectStylesheet({type, value}, sender) {
         if (!sender.tab) {
             return Promise.reject(new Error('Invalid tab'));
         }
 
         const tabId = sender.tab.id;
         const frameId = sender.frameId;
-        const details = {
-            code: css,
-            runAt: 'document_start',
-            cssOrigin: 'user',
-            allFrames: false
-        };
+        const details = (
+            type === 'file' ?
+            {
+                file: value,
+                runAt: 'document_start',
+                cssOrigin: 'author',
+                allFrames: false,
+                matchAboutBlank: true
+            } :
+            {
+                code: value,
+                runAt: 'document_start',
+                cssOrigin: 'user',
+                allFrames: false,
+                matchAboutBlank: true
+            }
+        );
         if (typeof frameId === 'number') {
             details.frameId = frameId;
         }
diff --git a/ext/bg/js/settings/popup-preview-frame.js b/ext/bg/js/settings/popup-preview-frame.js
index 8fd06222..e900d4e2 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();
     }
@@ -83,14 +84,13 @@ 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');
-        if (node !== null && outerStylesheet !== null) {
-            node.parentNode.insertBefore(outerStylesheet, node);
+        if (node !== null && result !== null) {
+            node.parentNode.insertBefore(result, node);
         }
 
         return result;
@@ -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 45203c03..59c46ab8 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -31,7 +31,6 @@ class Popup {
         this._visible = false;
         this._visibleOverride = null;
         this._options = null;
-        this._stylesheetInjectedViaApi = false;
         this._contentScale = 1.0;
         this._containerSizeContentScale = null;
 
@@ -158,21 +157,13 @@ class Popup {
         this._container.dataset.yomichanSiteColor = this._getSiteColor();
     }
 
-    async setCustomOuterCss(css, injectDirectly) {
-        // Cannot repeatedly inject stylesheets using web extension APIs since there is no way to remove them.
-        if (this._stylesheetInjectedViaApi) { return; }
-
-        if (injectDirectly || Popup._isOnExtensionPage()) {
-            Popup.injectOuterStylesheet(css);
-        } else {
-            if (!css) { return; }
-            try {
-                await apiInjectStylesheet(css);
-                this._stylesheetInjectedViaApi = true;
-            } catch (e) {
-                // NOP
-            }
-        }
+    async setCustomOuterCss(css, useWebExtensionApi) {
+        return await Popup._injectStylesheet(
+            'yomichan-popup-outer-user-stylesheet',
+            'code',
+            css,
+            useWebExtensionApi
+        );
     }
 
     setChildrenSupported(value) {
@@ -187,26 +178,6 @@ class Popup {
         return this._container.getBoundingClientRect();
     }
 
-    static injectOuterStylesheet(css) {
-        if (Popup.outerStylesheet === null) {
-            if (!css) { return; }
-            Popup.outerStylesheet = document.createElement('style');
-            Popup.outerStylesheet.id = 'yomichan-popup-outer-stylesheet';
-        }
-
-        const outerStylesheet = Popup.outerStylesheet;
-        if (css) {
-            outerStylesheet.textContent = css;
-
-            const par = document.head;
-            if (par && outerStylesheet.parentNode !== par) {
-                par.appendChild(outerStylesheet);
-            }
-        } else {
-            outerStylesheet.textContent = '';
-        }
-    }
-
     // Private functions
 
     _inject() {
@@ -248,10 +219,24 @@ class Popup {
             });
             this._observeFullscreen(true);
             this._onFullscreenChanged();
-            this.setCustomOuterCss(this._options.general.customPopupOuterCss, false);
+            this._injectStyles();
         });
     }
 
+    async _injectStyles() {
+        try {
+            await Popup._injectStylesheet('yomichan-popup-outer-stylesheet', 'file', '/fg/css/client.css', true);
+        } catch (e) {
+            // NOP
+        }
+
+        try {
+            await this.setCustomOuterCss(this._options.general.customPopupOuterCss, true);
+        } catch (e) {
+            // NOP
+        }
+    }
+
     _observeFullscreen(observe) {
         if (!observe) {
             this._fullscreenEventListeners.removeAllEventListeners();
@@ -526,15 +511,6 @@ class Popup {
         ];
     }
 
-    static _isOnExtensionPage() {
-        try {
-            const url = chrome.runtime.getURL('/');
-            return window.location.href.substring(0, url.length) === url;
-        } catch (e) {
-            // NOP
-        }
-    }
-
     static _getViewport(useVisualViewport) {
         const visualViewport = window.visualViewport;
         if (visualViewport !== null && typeof visualViewport === 'object') {
@@ -567,6 +543,80 @@ class Popup {
             bottom: window.innerHeight
         };
     }
+
+    static _isOnExtensionPage() {
+        try {
+            const url = chrome.runtime.getURL('/');
+            return window.location.href.substring(0, url.length) === url;
+        } catch (e) {
+            // NOP
+        }
+    }
+
+    static async _injectStylesheet(id, type, value, useWebExtensionApi) {
+        const injectedStylesheets = Popup._injectedStylesheets;
+
+        if (Popup._isOnExtensionPage()) {
+            // Permissions error will occur if trying to use the WebExtension API to inject
+            // into an extension page.
+            useWebExtensionApi = false;
+        }
+
+        let styleNode = injectedStylesheets.get(id);
+        if (typeof styleNode !== 'undefined') {
+            if (styleNode === null) {
+                // Previously injected via WebExtension API
+                throw new Error(`Stylesheet with id ${id} has already been injected using the WebExtension API`);
+            }
+        } else {
+            styleNode = null;
+        }
+
+        if (useWebExtensionApi) {
+            // Inject via WebExtension API
+            if (styleNode !== null && styleNode.parentNode !== null) {
+                styleNode.parentNode.removeChild(styleNode);
+            }
+
+            await apiInjectStylesheet(type, value);
+
+            injectedStylesheets.set(id, null);
+            return null;
+        }
+
+        // Create node in document
+        const parentNode = document.head;
+        if (parentNode === null) {
+            throw new Error('No parent node');
+        }
+
+        // Create or reuse node
+        const isFile = (type === 'file');
+        const tagName = isFile ? 'link' : 'style';
+        if (styleNode === null || styleNode.nodeName.toLowerCase() !== tagName) {
+            if (styleNode !== null && styleNode.parentNode !== null) {
+                styleNode.parentNode.removeChild(styleNode);
+            }
+            styleNode = document.createElement(tagName);
+            styleNode.id = id;
+        }
+
+        // Update node style
+        if (isFile) {
+            styleNode.rel = value;
+        } else {
+            styleNode.textContent = value;
+        }
+
+        // Update parent
+        if (styleNode.parentNode !== parentNode) {
+            parentNode.appendChild(styleNode);
+        }
+
+        // Add to map
+        injectedStylesheets.set(id, styleNode);
+        return styleNode;
+    }
 }
 
-Popup.outerStylesheet = null;
+Popup._injectedStylesheets = new Map();
diff --git a/ext/manifest.json b/ext/manifest.json
index 68a8adb4..b86459f9 100644
--- a/ext/manifest.json
+++ b/ext/manifest.json
@@ -30,7 +30,6 @@
             "fg/js/frontend.js",
             "fg/js/frontend-initialize.js"
         ],
-        "css": ["fg/css/client.css"],
         "match_about_blank": true,
         "all_frames": true
     }],
diff --git a/ext/mixed/js/api.js b/ext/mixed/js/api.js
index 86bdc73c..14900ecf 100644
--- a/ext/mixed/js/api.js
+++ b/ext/mixed/js/api.js
@@ -89,8 +89,8 @@ function apiFrameInformationGet() {
     return _apiInvoke('frameInformationGet');
 }
 
-function apiInjectStylesheet(css) {
-    return _apiInvoke('injectStylesheet', {css});
+function apiInjectStylesheet(type, value) {
+    return _apiInvoke('injectStylesheet', {type, value});
 }
 
 function apiGetEnvironmentInfo() {