--- a/widget/gtk/IMContextWrapper.cpp
+++ b/widget/gtk/IMContextWrapper.cpp
@@ -217,16 +217,123 @@ public:
                      NS_GET_A(aColor));
     }
     virtual ~GetTextRangeStyleText() {};
 };
 
 const static bool kUseSimpleContextDefault = false;
 
 /******************************************************************************
+ * SelectionStyleProvider
+ *
+ * IME (e.g., fcitx, ~4.2.8.3) may look up selection colors of widget, which
+ * is related to the window associated with the IM context, to support any
+ * colored widgets.  Our editor (like <input type="text">) is rendered as
+ * native GtkTextView as far as possible by default and if editor color is
+ * changed by web apps, nsTextFrame may swap background color of foreground
+ * color of composition string for making composition string is always
+ * visually distinct in normal text.
+ *
+ * So, we would like IME to set style of composition string to good colors
+ * in GtkTextView.  Therefore, this class overwrites selection colors of
+ * our widget with selection colors of GtkTextView so that it's possible IME
+ * to refer selection colors of GtkTextView via our widget.
+ ******************************************************************************/
+
+class SelectionStyleProvider final
+{
+public:
+    static SelectionStyleProvider* GetInstance()
+    {
+        if (sHasShutDown) {
+            return nullptr;
+        }
+        if (!sInstance) {
+            sInstance = new SelectionStyleProvider();
+        }
+        return sInstance;
+    }
+
+    static void Shutdown()
+    {
+      if (sInstance) {
+          g_object_unref(sInstance->mProvider);
+      }
+      delete sInstance;
+      sInstance = nullptr;
+      sHasShutDown = true;
+    }
+
+    // aGDKWindow is a GTK window which will be associated with an IM context.
+    void AttachTo(GdkWindow* aGDKWindow)
+    {
+        GtkWidget* widget = nullptr;
+        // gdk_window_get_user_data() typically returns pointer to widget that
+        // window belongs to.  If it's widget, fcitx retrieves selection colors
+        // of them.  So, we need to overwrite its style.
+        gdk_window_get_user_data(aGDKWindow, (gpointer*)&widget);
+        if (GTK_IS_WIDGET(widget)) {
+            gtk_style_context_add_provider(
+                gtk_widget_get_style_context(widget),
+                GTK_STYLE_PROVIDER(mProvider),
+                GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+        }
+    }
+
+    void OnThemeChanged()
+    {
+        // fcitx refers GtkStyle::text[GTK_STATE_SELECTED] and
+        // GtkStyle::bg[GTK_STATE_SELECTED] (although pair of text and *base*
+        // or *fg* and bg is correct).  gtk_style_update_from_context() will
+        // set these colors using the widget's GtkStyleContext and so the
+        // colors can be controlled by a ":selected" CSS rule.
+        nsAutoCString style(":selected{");
+        // FYI: LookAndFeel always returns selection colors of GtkTextView.
+        nscolor selectionForegroundColor;
+        if (NS_SUCCEEDED(LookAndFeel::GetColor(
+                             LookAndFeel::eColorID_TextSelectForeground,
+                             &selectionForegroundColor))) {
+            double alpha =
+              static_cast<double>(NS_GET_A(selectionForegroundColor)) / 0xFF;
+            style.AppendPrintf("color:rgba(%u,%u,%u,%f);",
+                               NS_GET_R(selectionForegroundColor),
+                               NS_GET_G(selectionForegroundColor),
+                               NS_GET_B(selectionForegroundColor), alpha);
+        }
+        nscolor selectionBackgroundColor;
+        if (NS_SUCCEEDED(LookAndFeel::GetColor(
+                             LookAndFeel::eColorID_TextSelectBackground,
+                             &selectionBackgroundColor))) {
+            double alpha =
+              static_cast<double>(NS_GET_A(selectionBackgroundColor)) / 0xFF;
+            style.AppendPrintf("background-color:rgba(%u,%u,%u,%f);",
+                               NS_GET_R(selectionBackgroundColor),
+                               NS_GET_G(selectionBackgroundColor),
+                               NS_GET_B(selectionBackgroundColor), alpha);
+        }
+        style.AppendLiteral("}");
+        gtk_css_provider_load_from_data(mProvider, style.get(), -1, nullptr);
+    }
+
+private:
+    static SelectionStyleProvider* sInstance;
+    static bool sHasShutDown;
+    GtkCssProvider* const mProvider;
+
+    SelectionStyleProvider()
+      : mProvider(gtk_css_provider_new())
+    {
+        OnThemeChanged();
+    }
+};
+
+SelectionStyleProvider* SelectionStyleProvider::sInstance = nullptr;
+bool SelectionStyleProvider::sHasShutDown = false;
+
+/******************************************************************************
  * IMContextWrapper
  ******************************************************************************/
 
 IMContextWrapper* IMContextWrapper::sLastFocusedContext = nullptr;
 bool IMContextWrapper::sUseSimpleContext;
 
 NS_IMPL_ISUPPORTS(IMContextWrapper,
                   TextEventDispatcherListener,
@@ -357,16 +464,21 @@ IMContextWrapper::GetIMName() const
 
 void
 IMContextWrapper::Init()
 {
     MozContainer* container = mOwnerWindow->GetMozContainer();
     MOZ_ASSERT(container, "container is null");
     GdkWindow* gdkWindow = gtk_widget_get_window(GTK_WIDGET(container));
 
+    // Overwrite selection colors of the window before associating the window
+    // with IM context since IME may look up selection colors via IM context
+    // to support any colored widgets.
+    SelectionStyleProvider::GetInstance()->AttachTo(gdkWindow);
+
     // NOTE: gtk_im_*_new() abort (kill) the whole process when it fails.
     //       So, we don't need to check the result.
 
     // Normal context.
     mContext = gtk_im_multicontext_new();
     gtk_im_context_set_client_window(mContext, gdkWindow);
     g_signal_connect(mContext, "preedit_changed",
         G_CALLBACK(IMContextWrapper::OnChangeCompositionCallback), this);
@@ -458,16 +570,23 @@ IMContextWrapper::Init()
          "PR_GetEnv(\"XMODIFIERS\")=\"%s\"",
          this, mOwnerWindow, mContext, nsAutoCString(im).get(),
          ToChar(mIsIMInAsyncKeyHandlingMode), ToChar(mIsKeySnooped),
          mSimpleContext, mDummyContext,
          gtk_im_multicontext_get_context_id(GTK_IM_MULTICONTEXT(mContext)),
          PR_GetEnv("XMODIFIERS")));
 }
 
+/* static */
+void
+IMContextWrapper::Shutdown()
+{
+    SelectionStyleProvider::Shutdown();
+}
+
 IMContextWrapper::~IMContextWrapper()
 {
     if (this == sLastFocusedContext) {
         sLastFocusedContext = nullptr;
     }
     MOZ_LOG(gGtkIMLog, LogLevel::Info,
         ("0x%p ~IMContextWrapper()", this));
 }
@@ -1361,16 +1480,26 @@ IMContextWrapper::OnSelectionChange(nsWi
         if (IsComposing() || retrievedSurroundingSignalReceived) {
             ResetIME();
         }
     }
 }
 
 /* static */
 void
+IMContextWrapper::OnThemeChanged()
+{
+    if (!SelectionStyleProvider::GetInstance()) {
+        return;
+    }
+    SelectionStyleProvider::GetInstance()->OnThemeChanged();
+}
+
+/* static */
+void
 IMContextWrapper::OnStartCompositionCallback(GtkIMContext* aContext,
                                              IMContextWrapper* aModule)
 {
     aModule->OnStartCompositionNative(aContext);
 }
 
 void
 IMContextWrapper::OnStartCompositionNative(GtkIMContext* aContext)

--- a/widget/gtk/IMContextWrapper.h
+++ b/widget/gtk/IMContextWrapper.h
@@ -43,32 +43,37 @@ public:
                           void* aData) override;
 
 public:
     // aOwnerWindow is a pointer of the owner window.  When aOwnerWindow is
     // destroyed, the related IME contexts are released (i.e., IME cannot be
     // used with the instance after that).
     explicit IMContextWrapper(nsWindow* aOwnerWindow);
 
+    // Called when the process is being shut down.
+    static void Shutdown();
+
     // "Enabled" means the users can use all IMEs.
     // I.e., the focus is in the normal editors.
     bool IsEnabled() const;
 
     // OnFocusWindow is a notification that aWindow is going to be focused.
     void OnFocusWindow(nsWindow* aWindow);
     // OnBlurWindow is a notification that aWindow is going to be unfocused.
     void OnBlurWindow(nsWindow* aWindow);
     // OnDestroyWindow is a notification that aWindow is going to be destroyed.
     void OnDestroyWindow(nsWindow* aWindow);
     // OnFocusChangeInGecko is a notification that an editor gets focus.
     void OnFocusChangeInGecko(bool aFocus);
     // OnSelectionChange is a notification that selection (caret) is changed
     // in the focused editor.
     void OnSelectionChange(nsWindow* aCaller,
                            const IMENotification& aIMENotification);
+    // OnThemeChanged is called when desktop theme is changed.
+    static void OnThemeChanged();
 
     /**
      * OnKeyEvent() is called when aWindow gets a native key press event or a
      * native key release event.  If this returns true, the key event was
      * filtered by IME.  Otherwise, this returns false.
      * NOTE: When the native key press event starts composition, this returns
      *       true but dispatches an eKeyDown event or eKeyUp event before
      *       dispatching composition events or content command event.

--- a/widget/gtk/mozgtk/mozgtk.c
+++ b/widget/gtk/mozgtk/mozgtk.c
@@ -519,16 +519,18 @@ STUB(gtk_window_set_title)
 STUB(gtk_window_set_transient_for)
 STUB(gtk_window_set_type_hint)
 STUB(gtk_window_set_wmclass)
 STUB(gtk_window_unfullscreen)
 STUB(gtk_window_unmaximize)
 #endif
 
 #ifdef GTK3_SYMBOLS
+STUB(gtk_css_provider_load_from_data)
+STUB(gtk_css_provider_new)
 STUB(gdk_device_get_source)
 STUB(gdk_device_manager_get_client_pointer)
 STUB(gdk_disable_multidevice)
 STUB(gdk_device_manager_list_devices)
 STUB(gdk_display_get_device_manager)
 STUB(gdk_display_manager_open_display)
 STUB(gdk_error_trap_pop_ignored)
 STUB(gdk_event_get_source_device)
@@ -562,16 +564,17 @@ STUB(gtk_render_frame_gap)
 STUB(gtk_render_handle)
 STUB(gtk_render_icon)
 STUB(gtk_render_line)
 STUB(gtk_render_option)
 STUB(gtk_render_slider)
 STUB(gtk_scale_new)
 STUB(gtk_scrollbar_new)
 STUB(gtk_style_context_add_class)
+STUB(gtk_style_context_add_provider)
 STUB(gtk_style_context_add_region)
 STUB(gtk_style_context_get)
 STUB(gtk_style_context_get_background_color)
 STUB(gtk_style_context_get_border)
 STUB(gtk_style_context_get_border_color)
 STUB(gtk_style_context_get_color)
 STUB(gtk_style_context_get_direction)
 STUB(gtk_style_context_get_margin)
@@ -588,16 +591,17 @@ STUB(gtk_style_context_remove_class)
 STUB(gtk_style_context_remove_region)
 STUB(gtk_style_context_restore)
 STUB(gtk_style_context_save)
 STUB(gtk_style_context_set_direction)
 STUB(gtk_style_context_set_path)
 STUB(gtk_style_context_set_parent)
 STUB(gtk_style_context_set_state)
 STUB(gtk_style_properties_lookup_property)
+STUB(gtk_style_provider_get_type)
 STUB(gtk_tree_view_column_get_button)
 STUB(gtk_widget_get_preferred_size)
 STUB(gtk_widget_get_preferred_width)
 STUB(gtk_widget_get_preferred_height)
 STUB(gtk_widget_get_state_flags)
 STUB(gtk_widget_get_style_context)
 STUB(gtk_widget_path_append_type)
 STUB(gtk_widget_path_copy)

--- a/widget/gtk/nsWidgetFactory.cpp
+++ b/widget/gtk/nsWidgetFactory.cpp
@@ -13,16 +13,17 @@
 #include "nsAppShellSingleton.h"
 #include "nsBaseWidget.h"
 #include "nsGtkKeyUtils.h"
 #include "nsLookAndFeel.h"
 #include "nsWindow.h"
 #include "nsTransferable.h"
 #include "nsHTMLFormatConverter.h"
 #include "HeadlessClipboard.h"
+#include "IMContextWrapper.h"
 #ifdef MOZ_X11
 #include "nsClipboardHelper.h"
 #include "nsClipboard.h"
 #include "nsDragService.h"
 #endif
 #ifdef MOZ_WIDGET_GTK
 #include "nsApplicationChooser.h"
 #endif
@@ -312,16 +313,17 @@ nsWidgetGtk2ModuleDtor()
   // Shutdown all XP level widget classes.
   WidgetUtils::Shutdown();
 
   NativeKeyBindings::Shutdown();
   nsLookAndFeel::Shutdown();
   nsFilePicker::Shutdown();
   nsSound::Shutdown();
   nsWindow::ReleaseGlobals();
+  IMContextWrapper::Shutdown();
   KeymapWrapper::Shutdown();
   nsGTKToolkit::Shutdown();
   nsAppShellShutdown();
 #ifdef MOZ_ENABLE_DBUS
   WakeLockListener::Shutdown();
 #endif
 }
 

--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -3438,16 +3438,18 @@ nsWindow::ThemeChanged()
 
         if (win && win != this) { // guard against infinite recursion
             RefPtr<nsWindow> kungFuDeathGrip = win;
             win->ThemeChanged();
         }
 
         children = children->next;
     }
+
+    IMContextWrapper::OnThemeChanged();
 }
 
 void
 nsWindow::OnDPIChanged()
 {
   if (mWidgetListener) {
     nsIPresShell* presShell = mWidgetListener->GetPresShell();
     if (presShell) {