No css file in docs or even in project

There is no css file in docs or npm files.
But if you install it, you can find css for ng2-select by using "code inspect" on demos page.
https://valor-software.com/ng2-select/assets/css/ng2-select.css

Don't forget to use the same classes.
Anyway, that lib is very old (last commit was 3 years ago), so you should try to find something new.