Box displayed around selected world cannot be retextured

The bug

Much like how the black "currently selected" box used for languages, resource packs and data packs cannot be retextured (MC-269438, MC-269443), the one used to indicate the currently selected world cannot be retextured either, and appears to be hardcoded.

How to fix

Introduce a texture file for this effect (an opaque black rectangle with a white border and an appropriate scaling mcmeta should suffice).