You should be able to restore the backup by removing the existing theme folder within cPanel file manager and replacing this theme folder with your backed up folder.
You can use backup plugins like BlogVault, UpdraftPlus, All-in-one WP Migration and etc. Also, you can take a backup from them via the dp-content/themes and replace these directories whenever you need to restore the theme. For downloading or uploading the directories and files, you can use FTP tools like FileZilla or WinSCP. Meantime, you can use web hosting backup/ restore solutions like JetBackup, Acronis Cyber Protect, etc. I hope this helps!