Does not work on admin pages

The description says:
“The CSS rules applied to the whole website will override the default stylesheets of your theme and plugins”

I think it is quite clear the plugin is not meant to work on admin pages.

Here you can find some useful snippets to add a new stylesheet to admin pages:
https://css-tricks.com/snippets/wordpress/apply-custom-css-to-admin-area/

I hope this help