[iOS] fixed dark mode for the discovery placeholder and updated guide cover placeholder

iphone/Maps/Images.xcassets/Discovery/img_guide_placeholder.imageset/Contents.json

@@ -2,7 +2,18 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "Illustration.pdf"
+      "filename" : "img_guides_gallery_placeholder.png",
+      "scale" : "1x"
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "img_guides_gallery_placeholder@2x.png",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "img_guides_gallery_placeholder@3x.png",
+      "scale" : "3x"
     }
   ],
   "info" : {

iphone/Maps/UI/Discovery/Table Cells/DiscoveryOnlineTemplateCell.swift

@@ -15,6 +15,7 @@ final class DiscoveryOnlineTemplateCell: MWMTableViewCell {
   
   @IBOutlet var containerView: UIView! {
     didSet {
+      containerView.backgroundColor = UIColor.white()
       containerView.layer.cornerRadius = 6.0
       containerView.layer.borderWidth = 1.0
       containerView.layer.borderColor = UIColor.blackDividers()?.cgColor
@@ -23,7 +24,11 @@ final class DiscoveryOnlineTemplateCell: MWMTableViewCell {
   @IBOutlet var header: UILabel!
   @IBOutlet var title: UILabel!
   @IBOutlet var subtitle: UILabel!
-  @IBOutlet var actionButton: UIButton!
+  @IBOutlet var actionButton: UIButton! {
+    didSet {
+      actionButton.setTitleColor(UIColor.linkBlue(), for: .normal)
+    }
+  }
 
   typealias Tap = () -> ()
   private var tap: Tap?