apply grub theme to submenus

1. split up the grub.cfg file
   • move the theme setting commands to a new file theme.cfg
   • move the other initial setup stuff to a new file config.cfg (this is not necessary to achieve the theme fix, but the idea in my upstream work where i'm also doing this within a pending chunk of work is to let users only replace the portions of a config they care about changing)
   • add a source /boot/grub/config.cfg and a source /boot/grub/theme.cfg line at the start of grub.cfg to import them
2. add source /boot/grub/theme.cfglines to every submenu to thus execute a command setting the theme

thus with the theme explicitly set in each submenu, it actually gets applied rather than submenus showing a default black theme with no splash.