use same font color for system messages as on Android

DcCore/DcCore/Helper/DcColors.swift

@@ -32,6 +32,7 @@ public struct DcColors {
     public static let providerPreparationBackground = UIColor.init(hexString: "fdf7b2")
     public static let providerBrokenBackground = UIColor.themeColor(light: SystemColor.red.uiColor, dark: SystemColor.red.uiColor)
     public static let systemMessageBackgroundColor = UIColor.init(hexString: "55444444")
+    public static let systemMessageFontColor = UIColor.themeColor(light: .white, dark: UIColor.init(hexString: "A4A6A9"))
     public static let deaddropBackground = UIColor.themeColor(light: UIColor.init(hexString: "ebebec"), dark: UIColor.init(hexString: "1a1a1c"))
 }
 

deltachat-ios/Chat/Views/Cells/InfoMessageCell.swift

@@ -19,6 +19,7 @@ class InfoMessageCell: UITableViewCell {
         label.lineBreakMode = .byWordWrapping
         label.textAlignment = .center
         label.font = UIFont.preferredFont(for: .subheadline, weight: .medium)
+        label.textColor = DcColors.systemMessageFontColor
         return label
     }()