fix: use theme primary color for stick headers

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

app/src/main/java/com/nextcloud/talk/contacts/ContactsActivityCompose.kt

@@ -185,7 +185,7 @@ fun Header(header: String) {
             .fillMaxSize()
             .background(Color.Transparent)
             .padding(start = 60.dp),
-        color = Color.Blue,
+        color = MaterialTheme.colorScheme.primary,
         fontWeight = FontWeight.Bold
     )
 }