📃 fix(grub-theme): theme backup

[koibtw]

Type of Change

• [x] Bug fix

Description

Backs up the (if) existing theme directory before downloading.

Issues / other PRs related

Resolves #739

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.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generate no errors/warnings/merge conflicts.

(📃 is a shell script btw)