app: fix terminal padding background color

src/app/app.scss

@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020-2021 The Pybricks Authors
+// Copyright (c) 2020-2022 The Pybricks Authors
 
 // Custom styling for the App control.
 
@@ -40,6 +40,7 @@
 
 .pb-app-terminal-padding {
     padding-left: 10px;
+    background-color: white;
 }
 
 // hide the docs and resize separator