Set new theme for date picker/calendar

It is necessary to create a custom theme for the date picker/calendar available from the Post Settings screen. This is because 'Theme.AppCompat.DayNight.Dialog.Alert' needs to be set as the theme's parent in order to inherit the other styles from that theme.

WordPress/src/main/java/org/wordpress/android/ui/posts/PostDatePickerDialogFragment.kt

@@ -32,6 +32,7 @@ class PostDatePickerDialogFragment : DialogFragment() {
 
         val datePickerDialog = DatePickerDialog(
                 requireContext(),
+                R.style.PostSettingsCalendar,
                 null,
                 viewModel.year ?: 0,
                 viewModel.month ?: 0,

WordPress/src/main/res/values/styles.xml

@@ -902,6 +902,11 @@
         <item name="colorControlActivated">@color/colorPrimaryEditor</item>
     </style>
 
+    <style name="PostSettingsCalendar" parent="Theme.AppCompat.DayNight.Dialog.Alert">
+        <item name="colorAccent">@color/colorPrimaryEditor</item>
+        <item name="colorControlActivated">@color/colorPrimaryEditor</item>
+    </style>
+
     <style name="PostSettingsCardViewInnerLayout">
         <item name="android:layout_width">match_parent</item>
         <item name="android:layout_height">wrap_content</item>