diff --git a/ext/bg/js/api.js b/ext/bg/js/api.js
index f768e6f9..94a70c34 100644
--- a/ext/bg/js/api.js
+++ b/ext/bg/js/api.js
@@ -241,3 +241,32 @@ function apiFrameInformationGet(sender) {
     const frameId = sender.frameId;
     return Promise.resolve({frameId});
 }
+
+function apiInjectStylesheet(css, 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
+    };
+    if (typeof frameId === 'number') {
+        details.frameId = frameId;
+    }
+
+    return new Promise((resolve, reject) => {
+        chrome.tabs.insertCSS(tabId, details, () => {
+            const e = chrome.runtime.lastError;
+            if (e) {
+                reject(new Error(e.message));
+            } else {
+                resolve();
+            }
+        });
+    });
+}
diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index 453f4282..c1216d95 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -185,6 +185,7 @@ Backend.messageHandlers = {
     screenshotGet: ({options}, sender) => apiScreenshotGet(options, sender),
     forward: ({action, params}, sender) => apiForward(action, params, sender),
     frameInformationGet: (params, sender) => apiFrameInformationGet(sender),
+    injectStylesheet: ({css}, sender) => apiInjectStylesheet(css, sender)
 };
 
 window.yomichan_backend = new Backend();
diff --git a/ext/fg/js/api.js b/ext/fg/js/api.js
index a553e514..dcfb2a09 100644
--- a/ext/fg/js/api.js
+++ b/ext/fg/js/api.js
@@ -64,3 +64,7 @@ function apiForward(action, params) {
 function apiFrameInformationGet() {
     return utilInvoke('frameInformationGet');
 }
+
+function apiInjectStylesheet(css) {
+    return utilInvoke('injectStylesheet', {css});
+}
diff --git a/ext/fg/js/popup.js b/ext/fg/js/popup.js
index 880e3efe..73ed37e0 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -38,6 +38,7 @@ class Popup {
         this.visible = false;
         this.visibleOverride = null;
         this.options = null;
+        this.stylesheetInjectedViaApi = false;
         this.updateVisibility();
     }
 
@@ -75,6 +76,7 @@ class Popup {
             });
             this.observeFullscreen();
             this.onFullscreenChanged();
+            this.setCustomOuterCss(this.options.general.customPopupOuterCss, false);
             this.isInjected = true;
         });
     }
@@ -334,6 +336,23 @@ class Popup {
         this.invokeApi('setCustomCss', {css});
     }
 
+    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
+            }
+        }
+    }
+
     clearAutoPlayTimer() {
         if (this.isInjected) {
             this.invokeApi('clearAutoPlayTimer');
@@ -375,4 +394,35 @@ class Popup {
     get url() {
         return window.location.href;
     }
+
+    static isOnExtensionPage() {
+        try {
+            const url = chrome.runtime.getURL('/');
+            return window.location.href.substr(0, url.length) === url;
+        } catch (e) {
+            // NOP
+        }
+    }
+
+    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 = '';
+        }
+    }
 }
+
+Popup.outerStylesheet = null;