added grub themes script

[guruswarupa]

Pull Request

added grub themes script

• grub bootloader theme script added.

Checklist

• [x] My code adheres to the coding and style guidelines of the project.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no errors/warnings/merge conflicts.