Adjust theme to future iOS 13 Dark Mode

Update provider logos where adequate:

- NordVPN (use white on dark)
- ProtonVPN (use green)

Passepartout-iOS/Global/Theme.swift

@@ -34,11 +34,21 @@ struct Theme {
         
         var accent1 = UIColor(rgb: 0xd69c68, alpha: 1.0)
 
-        var primaryText: UIColor = .darkText
+        var primaryText: UIColor = {
+            guard #available(iOS 13, *) else {
+                return .darkText
+            }
+            return .label
+        }()
         
         var primaryLightText: UIColor = .white
         
-        var secondaryText: UIColor = .gray
+        var secondaryText: UIColor = {
+            guard #available(iOS 13, *) else {
+                return .gray
+            }
+            return .secondaryLabel
+        }()
 
         var on: UIColor {
             return accent1

Passepartout-iOS/Providers.xcassets/nordvpn.imageset/Contents.json

@@ -4,11 +4,32 @@
       "idiom" : "universal",
       "scale" : "1x"
     },
+    {
+      "idiom" : "universal",
+      "scale" : "1x",
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ]
+    },
     {
       "idiom" : "universal",
       "filename" : "nordvpn@2x.png",
       "scale" : "2x"
     },
+    {
+      "idiom" : "universal",
+      "filename" : "nordvpn-dark@2x.png",
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ],
+      "scale" : "2x"
+    },
     {
       "idiom" : "universal",
       "filename" : "nordvpn@3x.png",