How can I reset my GRUB background to default?

A quick way to delete the file is --

Opening a terminal by CTRL + ALT + T and execute these command

sudo rm /usr/share/images/desktop-base/desktop-grub.png
sudo update-initramfs -u
sudo update-grub

How that file came

These files are from desktop-base package. That package's function as described in control file is

This package contains various miscellaneous files which are used by Debian Desktop installations. Currently, it provides some Debian-related artwork and themes, .desktop files containing links to Debian related material (suitable for placement on a user's desktop), and other common files between the available desktop environments such as GNOME and KDE.

It includes various wallpapers for both GNOME and EDE, splash screens etc. If you don't want that package, you can remove the package by this command

sudo apt-get purge desktop-base

then executes these commands to restore default grub screen

sudo update-initramfs -u
sudo update-grub

If you want to keep the splash files and want the default grub background

In that case, use the below command to disable checking for grub background file

 sudo chmod -x /etc/grub.d/05_debian_theme

and then execute

sudo update-initramfs -u 
sudo update-grub 

A simple way without deleting files or changing file mode bits is:

sudo nano /etc/default/grub

and insert

GRUB_BACKGROUND=""

then run

sudo update-grub