Adapt conversations color for #Forez

lib/models/config.dart

@@ -24,6 +24,7 @@ class Config {
   final String appName;
   final String appQuickDescription;
   final Color unreadConversationColor;
+  final Color defaultConversationColor;
 
   // Entries for the welcome tour
   final TourEntriesBuilder toursEntriesBuilder;
@@ -44,6 +45,7 @@ class Config {
     this.primaryColorDark,
     this.appName = "Comunic",
     this.appQuickDescription,
+    this.defaultConversationColor,
     this.unreadConversationColor,
     this.toursEntriesBuilder,
     this.additionalLoading,