What CSS framework is used in ExtJS?

Question

css

Hello.
The question is, what is the name of such a CSS framework? Or is it theirs?
I write in PHP, I would like to use it in my project. Documentation would be nice.

Answer 1

It's not a separate framework. This is a set of styles used only in ExtJS.
Something similar is here https://metroui.org.ua/examples/desktop/desktop.html

Answer 2

https://yuilibrary.com/ was taken as a basis , but the extJS developers have gone very far from it.

Answer 3

There is no css framework. Everything was written only for ExtJS. Forget in general.