How to make nav bar fixed on the top and the bottom and also how to make my entire page toggle dark and not just the top of the page

Code Help
1

Question:
Whenever I toggle my dark mode it only applies above the top navigation bar which also refuses to be in a fixed position
Repl link:
https://replit.com/@ElhadjiKeita/EKeitaAdvocayProject#index.html
https://replit.com/@ElhadjiKeita/EKeitaAdvocayProject#style.css