Firefox have couple of different ways to create themes. Theme as extension have advantage of the full range of features through theme API. When I start develop this project I choose to use extension approach if there is need for all features. After I build it, I find that only colors features is used and that is possible to just make Theme. Then I search and find that already exists couple of Nord Themes and I choose to stay be Extension so it is easy to differentiate from other. Beside that there are many users and I don't wanna make confusion.

If you wanna to use Theme I create it, under name "Nord Dark Theme"
https://addons.mozilla.org/en-US/firefox/addon/nord-dark-theme/