[CRD iOS] Fix color scheme and other UI issues

remoting/ios/app/pin_entry_view.mm

 // Copyright 2017 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.
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 #import "remoting/ios/app/pin_entry_view.h"
 
@@ skipping 28 matching lines @@
 
     _pairingLabel = [[UILabel alloc] init];
     _pairingLabel.textColor =
         [UIColor colorWithRed:1.f green:1.f blue:1.f alpha:0.5];
     _pairingLabel.font = [UIFont systemFontOfSize:12.f];
     _pairingLabel.text = @"Remember my PIN on this device.";
     [self addSubview:_pairingLabel];
 
     _pinButton =
         [MDCFloatingButton floatingButtonWithShape:MDCFloatingButtonShapeMini];
-    [_pinButton setTitle:@"+" forState:UIControlStateNormal];
+    [_pinButton setBackgroundColor:UIColor.whiteColor
+                          forState:UIControlStateNormal];
 
     // TODO(nicholss): Update "->" to the arrow icon.
     [_pinButton setTitle:@"->" forState:UIControlStateNormal];
     [_pinButton addTarget:self
                    action:@selector(didTapPinEntry:)
          forControlEvents:UIControlEventTouchUpInside];
     _pinButton.translatesAutoresizingMaskIntoConstraints = NO;
     [self addSubview:_pinButton];
 
     _pinEntry = [[UITextField alloc] init];
@@ skipping 54 matching lines @@
 }
 
 #pragma mark - Private
 
 - (void)didTapPinEntry:(id)sender {
   [_delegate didProvidePin:_pinEntry.text createPairing:_pairingSwitch.isOn];
   [_pinEntry endEditing:YES];
 }
 
 @end