use same background color for system messages as on Android

DcCore/DcCore/Helper/DcColors.swift

@@ -30,8 +30,7 @@ public struct DcColors {
                                                      dark: UIColor(red: 0.4, green: 0.4, blue: 0.4, alpha: 1))
     public static let providerPreparationBackground = UIColor.themeColor(lightHex: "#fffdf7b2", darkHex: "##fffdf7b2")
     public static let providerBrokenBackground = UIColor.themeColor(light: SystemColor.red.uiColor, dark: SystemColor.red.uiColor)
-    public static let systemMessageBackgroundColor = UIColor.themeColor(light: UIColor(white: 0.9, alpha: 0.5),
-                                                                        dark: UIColor(white: 0.2, alpha: 0.5))
+    public static let systemMessageBackgroundColor = UIColor.init(hexString: "55444444")
     public static let deaddropBackground = UIColor.themeColor(light: UIColor.init(hexString: "ebebec"), dark: UIColor.init(hexString: "1a1a1c"))
 }