The bug
this.drawDefaultBackground(); overrides the sign edit button.
Linked issues
is duplicated by 11
MC-137562
The "Done" button when editing a sign is incorrectly darkened with the rest of the background
Resolved
MC-137809
"Done" button when writing signs renders behind darkness
Resolved
MC-137821
Sign GUI is dark, darkening the Done button
Resolved
MC-138556
Sign 'Done' button is very dark
Resolved
MC-138597
Gray background when editing a sign overlays the "done" button.
Resolved
6 more links
can reproduce in 18w43c