# in /etc/default/grub change
GRUB_GFXMODE=1280x1024
# or your monitor's supported resolution
# if you have this line uncommented, comment it, like so:
# GRUB_TERMINAL=console

# I also suggest shutting off that super slow osprobe stuff, if you want, by adding this:
GRUB_DISABLE_OS_PROBER=true
# if you want the other OS in your system detected and added
# then don't add this

# if you don't want to see those gfx splash things as it boots
# remove the words quite splash from:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

#next, in file /etc/grub.d/00_header find the line starting with:
  set gfximode=
# after, add the line:
  set gfxmode=keep

update-grub