diff --git a/gulpfile.mjs b/gulpfile.mjs
index e79b79a96..008b5b99d 100644
--- a/gulpfile.mjs
+++ b/gulpfile.mjs
@@ -1431,6 +1431,7 @@ gulp.task(
           .pipe(
             postcss([
               discardCommentsCSS(),
+              postcssDarkThemeClass(),
               autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG),
             ])
           )
@@ -1441,6 +1442,7 @@ gulp.task(
           .pipe(
             postcss([
               discardCommentsCSS(),
+              postcssDarkThemeClass(),
               autoprefixer(MOZCENTRAL_AUTOPREFIXER_CONFIG),
             ])
           )
diff --git a/web/app.js b/web/app.js
index 52d58aeb4..2d23ca39a 100644
--- a/web/app.js
+++ b/web/app.js
@@ -202,39 +202,40 @@ const PDFViewerApplication = {
       await this._parseHashParams();
     }
 
-    if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
-      let mode;
-      switch (AppOptions.get("viewerCssTheme")) {
-        case 1:
-          mode = "is-light";
-          break;
-        case 2:
-          mode = "is-dark";
-          break;
+    let mode;
+    switch (AppOptions.get("viewerCssTheme")) {
+      case 1:
+        mode = "is-light";
+        break;
+      case 2:
+        mode = "is-dark";
+        break;
+    }
+    if (mode) {
+      document.documentElement.classList.add(mode);
+    }
+
+    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING")) {
+      if (AppOptions.get("enableFakeMLManager")) {
+        this.mlManager =
+          MLManager.getFakeMLManager?.({
+            enableGuessAltText: AppOptions.get("enableGuessAltText"),
+            enableAltTextModelDownload: AppOptions.get(
+              "enableAltTextModelDownload"
+            ),
+          }) || null;
       }
-      if (mode) {
-        document.documentElement.classList.add(mode);
+    } else if (PDFJSDev.test("MOZCENTRAL")) {
+      if (AppOptions.get("enableAltText")) {
+        // We want to load the image-to-text AI engine as soon as possible.
+        this.mlManager = new MLManager({
+          enableGuessAltText: AppOptions.get("enableGuessAltText"),
+          enableAltTextModelDownload: AppOptions.get(
+            "enableAltTextModelDownload"
+          ),
+          altTextLearnMoreUrl: AppOptions.get("altTextLearnMoreUrl"),
+        });
       }
-      if (typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING")) {
-        if (AppOptions.get("enableFakeMLManager")) {
-          this.mlManager =
-            MLManager.getFakeMLManager?.({
-              enableGuessAltText: AppOptions.get("enableGuessAltText"),
-              enableAltTextModelDownload: AppOptions.get(
-                "enableAltTextModelDownload"
-              ),
-            }) || null;
-        }
-      }
-    } else if (AppOptions.get("enableAltText")) {
-      // We want to load the image-to-text AI engine as soon as possible.
-      this.mlManager = new MLManager({
-        enableGuessAltText: AppOptions.get("enableGuessAltText"),
-        enableAltTextModelDownload: AppOptions.get(
-          "enableAltTextModelDownload"
-        ),
-        altTextLearnMoreUrl: AppOptions.get("altTextLearnMoreUrl"),
-      });
     }
 
     // Ensure that the `L10n`-instance has been initialized before creating
diff --git a/web/app_options.js b/web/app_options.js
index 0ccb5c3dd..6f768b316 100644
--- a/web/app_options.js
+++ b/web/app_options.js
@@ -334,6 +334,11 @@ const defaultOptions = {
     value: 1,
     kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
   },
+  viewerCssTheme: {
+    /** @type {number} */
+    value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("CHROME") ? 2 : 0,
+    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
+  },
   viewOnLoad: {
     /** @type {boolean} */
     value: 0,
@@ -489,11 +494,6 @@ if (typeof PDFJSDev === "undefined" || !PDFJSDev.test("MOZCENTRAL")) {
         : "../build/pdf.sandbox.mjs",
     kind: OptionKind.VIEWER,
   };
-  defaultOptions.viewerCssTheme = {
-    /** @type {number} */
-    value: typeof PDFJSDev !== "undefined" && PDFJSDev.test("CHROME") ? 2 : 0,
-    kind: OptionKind.VIEWER + OptionKind.PREFERENCE,
-  };
   defaultOptions.enableFakeMLManager = {
     /** @type {boolean} */
     value: true,