ios: Use main background color as the splash screen

Use `DesignVariables.mainBackground` as the iOS launch screen
colors.

Discussion:
  https://chat.zulip.org/#narrow/channel/530-mobile-design/topic/splash.20screen.20on.20iOS/near/2215883

Fixes: #1149

Author: rajveermalviya

ios/Runner.xcodeproj/project.pbxproj

@@ -12,7 +12,6 @@
 		74858FAF1ED2DC5600515810 /* AppDelegate.swift in Sources */ = {isa = PBXBuildFile; fileRef = 74858FAE1ED2DC5600515810 /* AppDelegate.swift */; };
 		97C146FC1CF9000F007C117D /* Main.storyboard in Resources */ = {isa = PBXBuildFile; fileRef = 97C146FA1CF9000F007C117D /* Main.storyboard */; };
 		97C146FE1CF9000F007C117D /* Assets.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = 97C146FD1CF9000F007C117D /* Assets.xcassets */; };
-		97C147011CF9000F007C117D /* LaunchScreen.storyboard in Resources */ = {isa = PBXBuildFile; fileRef = 97C146FF1CF9000F007C117D /* LaunchScreen.storyboard */; };
 		B34E9F092D776BEB0009AED2 /* Notifications.g.swift in Sources */ = {isa = PBXBuildFile; fileRef = B34E9F082D776BEB0009AED2 /* Notifications.g.swift */; };
 		F311C174AF9C005CE4AADD72 /* Pods_Runner.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 3EAE3F3F518B95B7BFEB4FE7 /* Pods_Runner.framework */; };
 /* End PBXBuildFile section */
@@ -47,7 +46,6 @@
 		97C146EE1CF9000F007C117D /* Runner.app */ = {isa = PBXFileReference; explicitFileType = wrapper.application; includeInIndex = 0; path = Runner.app; sourceTree = BUILT_PRODUCTS_DIR; };
 		97C146FB1CF9000F007C117D /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.storyboard; name = Base; path = Base.lproj/Main.storyboard; sourceTree = "<group>"; };
 		97C146FD1CF9000F007C117D /* Assets.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; path = Assets.xcassets; sourceTree = "<group>"; };
-		97C147001CF9000F007C117D /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.storyboard; name = Base; path = Base.lproj/LaunchScreen.storyboard; sourceTree = "<group>"; };
 		97C147021CF9000F007C117D /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		B34E9F082D776BEB0009AED2 /* Notifications.g.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Notifications.g.swift; sourceTree = "<group>"; };
 		B3AF53A72CA20BD10039801D /* Zulip.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; name = Zulip.xcconfig; path = Flutter/Zulip.xcconfig; sourceTree = "<group>"; };
@@ -112,7 +110,6 @@
 				3752899A2AF472D400475D9C /* Runner.entitlements */,
 				97C146FA1CF9000F007C117D /* Main.storyboard */,
 				97C146FD1CF9000F007C117D /* Assets.xcassets */,
-				97C146FF1CF9000F007C117D /* LaunchScreen.storyboard */,
 				97C147021CF9000F007C117D /* Info.plist */,
 				1498D2321E8E86230040F4C2 /* GeneratedPluginRegistrant.h */,
 				1498D2331E8E89220040F4C2 /* GeneratedPluginRegistrant.m */,
@@ -195,7 +192,6 @@
 			isa = PBXResourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
-				97C147011CF9000F007C117D /* LaunchScreen.storyboard in Resources */,
 				3B3967161E833CAA004F5970 /* AppFrameworkInfo.plist in Resources */,
 				97C146FE1CF9000F007C117D /* Assets.xcassets in Resources */,
 				97C146FC1CF9000F007C117D /* Main.storyboard in Resources */,
@@ -316,14 +312,6 @@
 			name = Main.storyboard;
 			sourceTree = "<group>";
 		};
-		97C146FF1CF9000F007C117D /* LaunchScreen.storyboard */ = {
-			isa = PBXVariantGroup;
-			children = (
-				97C147001CF9000F007C117D /* Base */,
-			);
-			name = LaunchScreen.storyboard;
-			sourceTree = "<group>";
-		};
 /* End PBXVariantGroup section */
 
 /* Begin XCBuildConfiguration section */

ios/Runner/AppDelegate.swift

@@ -10,6 +10,11 @@ import Flutter
     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
   ) -> Bool {
     GeneratedPluginRegistrant.register(with: self)
+
+    // Use `DesignVariables.mainBackground` color as the background color
+    // of the default UIView.
+    window?.backgroundColor = UIColor(named: "LaunchBackground");
+
     let controller = window?.rootViewController as! FlutterViewController
 
     // Retrieve the remote notification payload from launch options;

ios/Runner/Assets.xcassets/LaunchBackground.colorset/Contents.json

@@ -0,0 +1,38 @@
+{
+  "colors" : [
+    {
+      "color" : {
+        "color-space" : "srgb",
+        "components" : {
+          "alpha" : "1.000",
+          "blue" : "0xF0",
+          "green" : "0xF0",
+          "red" : "0xF0"
+        }
+      },
+      "idiom" : "universal"
+    },
+    {
+      "appearances" : [
+        {
+          "appearance" : "luminosity",
+          "value" : "dark"
+        }
+      ],
+      "color" : {
+        "color-space" : "srgb",
+        "components" : {
+          "alpha" : "1.000",
+          "blue" : "0x1D",
+          "green" : "0x1D",
+          "red" : "0x1D"
+        }
+      },
+      "idiom" : "universal"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

ios/Runner/Assets.xcassets/LaunchImage.imageset/Contents.json

@@ -52,8 +52,11 @@
 		<string>fetch</string>
 		<string>remote-notification</string>
 	</array>
-	<key>UILaunchStoryboardName</key>
-	<string>LaunchScreen</string>
+	<key>UILaunchScreen</key>
+	<dict>
+		<key>UIColorName</key>
+		<string>LaunchBackground</string>
+	</dict>
 	<key>UIMainStoryboardFile</key>
 	<string>Main</string>
 	<key>UISupportedInterfaceOrientations</key>