How to add global style to angular 6/7 library

I have a workaround for this. Just create the root component of your library without view encapsulation and all its styles will be then global.

my-library.component.ts

import { Component, OnInit, ViewEncapsulation } from '@angular/core';

@Component({
  selector: 'lib-my-library',
  templateUrl: './my-library.component.html',
  styleUrls: ['./my-library.component.scss'],
  encapsulation: ViewEncapsulation.None
})
export class MyLibraryComponent implements OnInit {

  constructor() { }

  ngOnInit() {
  }

}

my-library.component.html

<!-- html content -->

my-library.component.scss

@import './styles/core.scss';

Now your my-library.component.scss and core.scss are global

styles/core.scss

body {
    background: #333;
}

core.scss is optional, I just like to keep the root files clean.

Update: In case you want your mixins and variables too, then follow this answer.

As @codeepic already pointed out, there is currently a standard solution.

In ng-package.json add

"assets": ["./styles/**/*.css"]

The provided paths should be the paths to your files. At the same time, they will be the paths inside your /dist folder.
On build, the files will be copied to /dist. Users of your library will be able to add them to their global styles as follows.

/* styles.css */
@import url('node_modules/<your-library-name>/styles/<file-name>');

This way you can copy any type of files.

P.S. When used with CSS, do not forget that you can create an index.css file that can be imported just like node_modules/<your-library-name>/styles.