Although I welcome your idea, because I also love this theme and wished for a 4:3 version for a long time, I have to ask if you asked the original author lipebello if he's okay with it? At least you should credit him here and in your repo, and maybe link to the original theme at
https://github.com/lipebello/es-theme-retrorama.
I mention this because nearly a year ago, the widespread misuse of his work, sometimes even with his name removed from it, angered him so much that he removed his work from Github and swore never to make any of his work public again. Only the immense encouragement from his fans got him back on track. You can read his statement and the following pleas from his fans in the Retropie forum:
https://retropie.org.uk/forum/post/131800 (post by lipebello on 10 Feb 2018, 14:27)
So, it may be advisible to contact him about your project, lest we may lose him again, probably for good this time.
That said, I wish you the best and that lipebello is okay with your fork of his great theme.
(edit: just a typo)