There is a way to display boot messages in terminal, without any theme changes:
sudo gedit /etc/default/grub
Find this line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
And change it to:
GRUB_CMDLINE_LINUX_DEFAULT=""
Or even more boot messages:
GRUB_CMDLINE_LINUX_DEFAULT="no_console_suspend initcall_debug"
To change background add this line: (or install grub theme)
GRUB_BACKGROUND=”/pathtofolder/picture.png”
Then execute the following command and reboot the system:
sudo update-grub
Note:
If we want to change the Background of grub menu, the image should be
PNG, JPG or TGA image. JPG/ JPEG images must be 8-bit (256 color).
Else you will get errors, so the PNG images are preferable. Images
should be non-indexed, RGB.
The GIMP image editor is one application which can edit images to
conform to the GRUB 2 standards. Use the Image > Mode menu options to
set the properties to RGB and ensure the mode is not set to Indexed