the theme needs explicitly applying for submenus otherwise they are just
displayed with a default layout with a black background.
with the theme setting commands moved to a separate file, we can now
simply `source` that into each submenu.
this of course assumes that live-theme/theme.txt is suitable for submenus.
as done in upstream live-build MR #135.
the intention here is to prepare to fix application of the theme for submenus,
as addressed next.
we could accomplish that just by the theme.cfg portion of the split, but why
not do the config.cfg bit also now. this is similar to how syslinux is broken
up, and a part of the live-build MR #135 changes are to merge config/bootloader
files on top of the default set, rather than use them instead, thus
user/derivative configs only need to carry the bootloader files they actually
want to replace. breaking up grub.cfg helps towards this, so eventually kali
can drop carrying files that are identical to upstream ilve-build's.
live-build commit 24aab15d2a133075992ce3a216b1c3476dc7bd4c has added
generation of speech synth entries. this ensures that with the next
release of live-build, the kali one is not added as a duplicate.
we have to take a different approach here than for syslinux. the approach
relies upon an expectation that in the next live-build release the grub
config will include an install.cfg file containing install entries, like
syslinux, as a result of changes awaiting merge in MR #135 ([1]).
[1]: https://salsa.debian.org/live-team/live-build/-/merge_requests/135