Fixed issues Add some icons

What's Changed

• Pull request for 149'th issue by @The-Plottwist in https://github.com/vinceliuice/grub2-themes/pull/151
• Added generate hook by @dmadisetti in https://github.com/vinceliuice/grub2-themes/pull/155
• Add NixOS support by @dmadisetti in https://github.com/vinceliuice/grub2-themes/pull/158
• Update install.sh to fix installation on Fedora 34 and later. by @PatrickJosh in https://github.com/vinceliuice/grub2-themes/pull/161
• Make splashImage option a nullOr type by @thomassdk in https://github.com/vinceliuice/grub2-themes/pull/162
• Added theme variant name to the preview image by @Rdkang in https://github.com/vinceliuice/grub2-themes/pull/175
• Fix alignments of icons and add ubuntuDDE by @dotlogix in https://github.com/vinceliuice/grub2-themes/pull/178
• Remove override of THEME_DIR by @hyttmi in https://github.com/vinceliuice/grub2-themes/pull/180

New Contributors

• @The-Plottwist made their first contribution in https://github.com/vinceliuice/grub2-themes/pull/151
• @dmadisetti made their first contribution in https://github.com/vinceliuice/grub2-themes/pull/155