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).
Linked issues
relates to 1
Attachments
Comments 6

Can confirm. Possibly invalid provided the resolution of MC-269437 denoted in the description.

As Jiingy said, this is likely invalid due to the fact being this duplicates MC-269437 so its either a duplicate or invalid.
Not a duplicate, the two issues describe different assets which can't be modified. As for invalid, that's for Mojang to decide.


Can confirm in 24w14a and 1.20.5 pre-release 1.
I don't really think this is a bug. Only a problem, and I think the reason you're experiencing this problem is because nobody cares about these small details, so you can search for texture packs that retexture world selection box.