You can uninstall or removes an installed grub2-themes-ubuntu-mate package itself from Ubuntu 16.04 LTS (Xenial Xerus) through the terminal,
$ sudo apt-get remove grub2-themes-ubuntu-mate
If you would like to remove grub2-themes-ubuntu-mate and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove grub2-themes-ubuntu-mate
If you use with purge options to grub2-themes-ubuntu-mate package all the configuration and dependent packages will be removed.
$ sudo apt-get purge grub2-themes-ubuntu-mate
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove grub2-themes-ubuntu-mate
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