dialog buttons shouldn't have a background color

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

src/main/java/com/owncloud/android/ui/fragment/contactsbackup/ContactsBackupFragment.java

@@ -449,9 +449,13 @@ public class ContactsBackupFragment extends FileFragment implements DatePickerDi
 
             datePickerDialog.setTitle("");
             datePickerDialog.show();
-            // TODO How do we deal with primary colours that are too light or dark
-            datePickerDialog.getButton(DatePickerDialog.BUTTON_NEGATIVE).setTextColor(ThemeUtils.primaryColor(getContext()));
-            datePickerDialog.getButton(DatePickerDialog.BUTTON_POSITIVE).setTextColor(ThemeUtils.primaryColor(getContext()));
+
+            datePickerDialog.getButton(DatePickerDialog.BUTTON_NEGATIVE).setTextColor(ThemeUtils.primaryColor(getContext(),true));
+            datePickerDialog.getButton(DatePickerDialog.BUTTON_POSITIVE).setTextColor(ThemeUtils.primaryColor(getContext(), true));
+
+            // set background to transparent
+            datePickerDialog.getButton(DatePickerDialog.BUTTON_NEGATIVE).setBackgroundColor(0x00000000);
+            datePickerDialog.getButton(DatePickerDialog.BUTTON_POSITIVE).setBackgroundColor(0x00000000);
         } else {
             DisplayUtils.showSnackMessage(getView().findViewById(R.id.contacts_linear_layout),
                     R.string.contacts_preferences_something_strange_happened);