diff -up firefox-48.0/modules/libpref/init/all.js.orig firefox-48.0/modules/libpref/init/all.js
--- firefox-48.0/modules/libpref/init/all.js.orig	2016-07-25 22:22:05.000000000 +0200
+++ firefox-48.0/modules/libpref/init/all.js	2016-07-28 13:14:39.065888837 +0200
@@ -4497,6 +4497,7 @@ pref("gfx.apitrace.enabled",false);
 pref("gfx.content.use-native-pushlayer", true);
 #ifdef MOZ_WIDGET_GTK
 pref("gfx.xrender.enabled",false);
+pref("widget.allow-gtk-dark-theme", false);
 #endif
 #endif
 
diff -up firefox-48.0/widget/gtk/nsLookAndFeel.cpp.orig firefox-48.0/widget/gtk/nsLookAndFeel.cpp
--- firefox-48.0/widget/gtk/nsLookAndFeel.cpp.orig	2016-06-01 06:11:44.000000000 +0200
+++ firefox-48.0/widget/gtk/nsLookAndFeel.cpp	2016-07-28 13:58:35.645067880 +0200
@@ -28,6 +28,7 @@
 #include <dlfcn.h>
 
 #include "mozilla/gfx/2D.h"
+#include "mozilla/Preferences.h"
 
 #if MOZ_WIDGET_GTK != 2
 #include <cairo-gobject.h>
@@ -1097,16 +1098,22 @@ nsLookAndFeel::Init()
     // with wrong color theme, see Bug 972382
     GtkSettings *settings = gtk_settings_get_for_screen(gdk_screen_get_default());
 
-    // Disable dark theme because it interacts poorly with widget styling in
-    // web content (see bug 1216658).
-    // To avoid triggering reload of theme settings unnecessarily, only set the
-    // setting when necessary.
-    const gchar* dark_setting = "gtk-application-prefer-dark-theme";
-    gboolean dark;
-    g_object_get(settings, dark_setting, &dark, nullptr);
 
-    if (dark) {
-        g_object_set(settings, dark_setting, FALSE, nullptr);
+    bool isContentProcess = XRE_IsContentProcess();
+    bool allowDarkTheme = mozilla::Preferences::GetBool("widget.allow-gtk-dark-theme", false);
+
+    if (isContentProcess || !allowDarkTheme) {
+        // Disable dark theme because it interacts poorly with widget styling in
+        // web content (see bug 1216658).
+        // To avoid triggering reload of theme settings unnecessarily, only set the
+        // setting when necessary.
+        const gchar* dark_setting = "gtk-application-prefer-dark-theme";
+        gboolean dark;
+        g_object_get(settings, dark_setting, &dark, nullptr);
+
+        if (dark) {
+            g_object_set(settings, dark_setting, FALSE, nullptr);
+        }
     }
 
     GtkWidgetPath *path = gtk_widget_path_new();