Find your monitor's resolution and copy the corresponding folder to /boot/grub/themes
Edit your /etc/default/grub
file to include GRUB_THEME="/boot/grub/themes/ *folder you copied* /theme.txt"
For example: GRUB_THEME="/boot/grub/themes/yorha-1920x1080/theme.txt"
Finalize your changes with sudo update-grub