The bug
Advancements are connected by white lines with black borders. These do not appear to be retexturable, unlike other facets of the advancement menu.
How to fix
Introduce dedicated texture files for these branches.
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.