The bug
When using an anvil to modify an item, the experience cost will be displayed with a darker gray box behind it. This darker gray box cannot be retextured at all.
How to fix
Introduce a texture file for this gray box and make the anvil user interface use it.
Linked issues
relates to 25
MC-278413
Chat background cannot be retextured
Open
MC-268805
panorama_overlay.png no longer works
Resolved
MC-268927
The dark background in inventory screens cannot be changed via resource packs
Resolved
MC-269079
Death screen background cannot be retextured
Resolved
MC-269364
Sign editing screen can no longer be retextured
Resolved
20 more links
Can confirm.