Update - Newest Way

Don't do it, if you can avoid it. As Devon Sans points out in the comments: This feature will most likely be deprecated.

#Update - Newer WayFrom Angular 4.3.0, all piercing css combinators were deprecated. Angular team introduced a new combinator ::ng-deep (still it is experimental and not the full and final way) as shown below,

DEMO : https://plnkr.co/edit/RBJIszu14o4svHLQt563?p=preview

styles: [    `     :host { color: red; }          :host ::ng-deep parent {       color:blue;     }     :host ::ng-deep child{       color:orange;     }     :host ::ng-deep child.class1 {       color:yellow;     }     :host ::ng-deep child.class2{       color:pink;     }    `],template: `      Angular2                                //red      <parent>                                //blue          <child></child>                     //orange          <child class="class1"></child>      //yellow          <child class="class2"></child>      //pink      </parent>          `

You can use encapsulation mode and/or piercing CSS combinators >>>, /deep/ and ::shadow

working example : http://plnkr.co/edit/1RBDGQ?p=preview

styles: [    `     :host { color: red; }     :host >>> parent {       color:blue;     }     :host >>> child{       color:orange;     }     :host >>> child.class1 {       color:yellow;     }     :host >>> child.class2{       color:pink;     }    `    ],template: `  Angular2                                //red  <parent>                                //blue      <child></child>                     //orange      <child class="class1"></child>      //yellow      <child class="class2"></child>      //pink  </parent>      `

You should NOT use ::ng-deep, it is deprecated. In Angular, the proper way to change the style of children's component from the parent is to use encapsulation (read the warning below to understand the implications):

import { ViewEncapsulation } from '@angular/core';@Component({    ....    encapsulation: ViewEncapsulation.None})

And then, you will be able to modify the css form your component without a need from ::ng-deep

.mat-sort-header-container {  display: flex;  justify-content: center;}

WARNING: Doing this will make all css rules you write for this component to be global.

In order to limit the scope of your css to this component and his child only, add a css class to the top tag of your component and put your css "inside" this tag:

template:    <div class='my-component'>      <child-component class="first">First</child>    </div>,

Scss file:

.my-component {  // All your css goes in there in order not to be global}

UPDATE 3:

::ng-deep is also deprecated which means you should not do this at all anymore. It is unclear how this affects things where you need to override styles in child components from a parent component. To me it seems odd if this gets removed completely because how would this affect things as libraries where you need to override styles in a library component?

Comment if you have any insight in this.

UPDATE 2:

Since /deep/ and all other shadow piercing selectors are now deprecated. Angular dropped ::ng-deep which should be used instead for a broader compatibility.

UPDATE:

If using Angular-CLI you need to use /deep/ instead of >>> or else it will not work.

ORIGINAL:

After going to Angular2's Github page and doing a random search for "style" I found this question: Angular 2 - innerHTML styling

Which said to use something that was added in 2.0.0-beta.10, the >>> and ::shadow selectors.

(>>>) (and the equivalent/deep/) and ::shadow were added in 2.0.0-beta.10. They are similar to the shadow DOM CSS combinators (which are deprecated) and only work with encapsulation: ViewEncapsulation.Emulated which is the default in Angular2. They probably also work with ViewEncapsulation.None but are then only ignored because they are not necessary. These combinators are only an intermediate solution until more advanced features for cross-component styling is supported.

So simply doing:

:host >>> .child {}

In parent's stylesheet file solved the issue. Please note, as stated in the quote above, this solution is only intermediate until more advanced cross-component styling is supported.