It took me a while to figure out how to remove the grub theme.
It is too slow. When I boot, I want to hit the enter key right when it pops up. (so does everyone). I have to wait. And if I want to choose another selection it is so painful to do.
On the bright side, I discovered grub2 themes while learning how to get my old grub back.
Can you explain the procedure you followed to change the grub theme?
I googled it and found it. The difficult part was to find out it was a theme.
Now it was easy to find.
sudo apt-get remove grub2-themes-ubuntu-mate
I usually don’t put much weight on solving VirtualBox graphics problems but yeah, those grub themes muck up the text display in a VM pretty bad. In particular when I try to view the “Advanced options” screen the text is not framed properly and much is outside the border.
But removing grub2-themes-ubuntu-mate did the trick!
VirtualBox hasn’t had a good history with grub graphics modes anyway. I recall several versions ago VBox seemed to be bit-banging for 90 seconds JUST to display one of those useless fancy grub screens for some (not Ubuntu) distro. I plan on fighting my way back to text mode and forget that stuff! 
I recently reinstalled (setting ibus permanently crashed my synapse). I went with a uefi install instead. The theme is quick and quite good. I wonder why the legacy grub is so different and useless.
BIOS( legacy grub) is pretty much outdated and UEFI
offers many useful features. Thus it is recommended to install any
operating system in UEFI mode.