quote background image pathname in 00_header and 05_debian_theme output

This is required if the pathname contains spaces or grub shell metacharacters else the generated config file check will fail.

Patch for grub.d/00_header submitted to upstream: https://lists.gnu.org/archive/html/grub-devel/2024-05/msg00191.html

(I cannot believe this was not spotted before - or was noone fool enough to use a pathname with spaces and metacharacters until yesterday ?)