Fix UI style of signin AlertDialogs on Android

chrome/android/java/res/values-v17/styles.xml

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Copyright 2014 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file. -->
 
 <resources xmlns:tools="http://schemas.android.com/tools">
     <!-- Q: Why put style resources under values-v17/ ?
          A: Problem:
             1. paddingStart causes a crash on Galaxy Tab&Note b/8351339.
             2. So we wrote a build script(generate_v14_compatible_resources.py) to convert
@@ skipping 401 matching lines @@
         <item name="android:textColor">@color/default_text_color</item>
         <item name="android:textColorLink">@color/light_active_color</item>
         <item name="colorPrimaryDark">@android:color/black</item>
         <item name="colorAccent">@color/light_active_color</item>
 
         <!-- Remove ActionBar -->
         <item name="windowNoTitle">true</item>
         <item name="windowActionBar">false</item>
     </style>
 
+    <style name="SigninAlertDialogTheme" parent="AlertDialogTheme">
+        <item name="buttonBarButtonStyle">@style/SigninDialogButtonStyle</item>
+        <item name="android:textColorPrimary">@color/default_text_color</item>
+    </style>
+
+    <style name="SigninDialogButtonStyle" parent="Widget.AppCompat.Button.ButtonBar.AlertDialog">
+        <item name="android:textColor">@color/light_active_color</item>
+    </style>
+
     <!-- Bookmarks -->
     <style name="BookmarkTitleBarButton">
         <item name="android:layout_width">wrap_content</item>
         <item name="android:layout_height">wrap_content</item>
         <item name="android:background">@null</item>
         <item name="android:padding">15dp</item>
     </style>
     <style name="BookmarkMenuStyle" parent="Widget.AppCompat.ListPopupWindow">
         <item name="android:popupBackground">@drawable/menu_bg</item>
     </style>
@@ skipping 244 matching lines @@
         <item name="android:contentDescription">@null</item>
         <item name="android:scaleType">center</item>
         <item name="android:background">@color/light_active_color</item>
     </style>
 
     <!-- Miscellaneous -->
     <style name="BottomSheetContentTitle" parent="TextAppearance.Widget.AppCompat.Toolbar.Title">
         <item name="android:textSize">20sp</item>
     </style>
 </resources>