i dont have any documentation specific for custom UI, but few discussions which gave me some idea on how to create an app are,
https://frappe.io/docs/user/en/tutorial/new-app
Basically what i did was create a new app from bench using,
bench new-app app name
It created few files. And then i added my css in the public/css folder within the new app. The css path was also added on hooks.py
Then i installed the app to my site using,
bench --site sitename install-app appname.
Voila…
I will be posting link to the files once its on github.