Make sure not to miss the explanation of :host-context which is directly above ::ng-deep in the angular guide : https://angular.io/guide/component-styles. I missed it up until now and wish I'd seen it sooner.
::ng-deep is often necessary when you didn't write the component and don't have access to its source, but :host-context can be a very useful option when you do.
For example I have a black <h1> header inside a component I designed, and I want the ability to change it to white when it's displayed on a dark themed background.
If I didn't have access to the source I may have to do this in the css for the parent:
.theme-dark widget-box ::ng-deep h1 { color: white; }
But instead with :host-context you can do this inside the component.
h1
{
color: black; // default color
:host-context(.theme-dark) &
{
color: white; // color for dark-theme
}
// OR set an attribute 'outside' with [attr.theme]="'dark'"
:host-context([theme='dark']) &
{
color: white; // color for dark-theme
}
}
This will look anywhere in the component chain for .theme-dark and apply the css to the h1 if found. This is a good alternative to relying too much on ::ng-deep which while often necessary is somewhat of an anti-pattern.
In this case the & is replaced by the h1 (that's how sass/scss works) so you can define your 'normal' and themed/alternative css right next to each other which is very handy.
Be careful to get the correct number of :. For ::ng-deep there are two and for :host-context only one.