tile palette background color
EditorGUILayout.BeginHorizontal();
float leftMargin = (Screen.width / EditorGUIUtility.pixelsPerPoint - TilemapEditorTool.tilemapEditorToolsToolbarSize) * 0.5f;
GUILayout.Space(leftMargin);
EditorGUILayout.EditorToolbar(TilemapEditorTool.tilemapEditorTools);
previewUtility.camera.backgroundColor = EditorGUILayout.ColorField(previewUtility.camera.backgroundColor); // <--- CHANGE!
GUILayout.Space(leftMargin);
EditorGUILayout.EndHorizontal();