No idea whether this is the canonical method, but I created a file config/bootloaders/grub-pc/grub.cfg, and put my configuration there.
For example:
-----------------
insmod all_video
set menu_color_normal=cyan/blue
set menu_color_highlight=white/blue
menuentry --hotkey=l 'Live System' {
linux /live/vmlinuz boot=live components timezone=America/New_York
initrd /live/initrd.img
}
menuentry --hotkey=a 'Live from RAM' {
linux /live/vmlinuz boot=live components timezone=America/New_York toram
initrd /live/initrd.img
}
menuentry --hotkey=f 'Live Failsafe' {
set gfxpayload=auto
linux /live/vmlinuz boot=live components noapic noapm nodma noapic noapm nodma nomcce nolapic nomodeset nosmp nosplash nox11autologin timezone=America/New_York
initrd /live/initrd.img
}
menuentry "memtest86+" {
linux16 /live/memtest
}
-----------------
Works for me.
--Juergen