grub2: fix application of theme to submenus

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.

kali-config/common/bootloaders/grub-pc/grub.cfg

@@ -26,6 +26,8 @@ menuentry "Start installer with speech synthesis" {
 
 submenu 'Advanced options...' {
 
+source /boot/grub/theme.cfg
+
 # More installer entries (if any)
 LINUX_ADVANCED_INSTALL