For things like this GRUB_HIDDEN_TIMEOUT customisation, please use files of the form /etc/default/grub.d/<some name>.cfg rather than /etc/grub.d/00_before_header.
For things like this GRUB_HIDDEN_TIMEOUT customisation, please use files of the form /etc/default/ grub.d/ <some name>.cfg rather than /etc/grub. d/00_before_ header.