Box displayed around selected language cannot be retextured

Discovered while testing MC-269437.

The bug

The black box which is displayed around the currently active language in the language menu does not appear to be retexturable.

I had initially assumed, due to the black center and white border, that this used either the text_field or text_field_highlighted widget textures, but changing these via resource pack has no effect on it.

How to reproduce

1. Download and apply the attached resource pack

2. Enter the language menu

Expected results

The box shown behind the current language would look different from how it usually does.

Actual results

It remains completely unchanged.

How to fix

Instead of pointing this to the text field texture, it may be better to use a different, dedicated texture for this instead, since it isn't an editable text box.