Fix expand thread cell using wrong background color

Closes #383

Tusker/Screens/Conversation/ExpandThreadCollectionViewCell.swift

@@ -144,7 +144,15 @@ class ExpandThreadCollectionViewCell: UICollectionViewListCell {
     }
     
     override func updateConfiguration(using state: UICellConfigurationState) {
-        backgroundConfiguration = .appListPlainCell(for: state)
+        var config = UIBackgroundConfiguration.listGroupedCell().updated(for: state)
+        if state.isFocused {
+            // use default
+        } else if state.isHighlighted || state.isSelected {
+            config.backgroundColor = .appSelectedCellBackground
+        } else {
+            config.backgroundColor = .appSecondaryBackground
+        }
+        backgroundConfiguration = config
     }
     
 }