docs(styles): add guide for overriding styles (#6541)

guides/customizing-component-styles.md

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+# Customizing Angular Material component styles
+
+### Styling concepts
+
+There are 3 questions to keep in mind while customizing the styles of Angular Material
+components:
+
+1. Are your styles encapsulated?
+2. Are your styles more specific than the defaults?
+3. Is the component a child of your component, or does it exist elsewhere in the DOM?
+
+##### View encapsulation
+
+By default, Angular component styles are scoped to affect the component's view. This means that
+the styles you write will affect all the elements in your component template. They will *not*
+affect elements that are children of other components within your template. You can read more
+about view encapsulation in the
+[Angular documentation](https://angular.io/guide/component-styles#view-encapsulation). You may
+also wish to take a look at
+[_The State of CSS in Angular_](https://blog.angular.io/the-state-of-css-in-angular-4a52d4bd2700)
+on the Angular blog.
+
+##### Selector specificity
+
+Each CSS declaration has a level of *specificity* based on the type and number of selectors used.
+More specific styles will take precedence over less specific styles. Angular Material uses the
+least specific selectors possible for its components in order to make it easy to override them.
+You can read more about specificity and how it is calculated on the
+[MDN web docs](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity).
+
+##### Component location
+
+Some Angular Material components, specifically overlay-based ones like MdDialog, MdSnackbar, etc.,
+do not exist as children of your component. Often they are injected elsewhere in the DOM. This is
+important to keep in mind, since even using high specificity and shadow-piercing selectors will
+not target elements that are not direct children of your component. Global styles are recommended
+for targeting such components.
+
+### Styling overlay components
+
+Overlay-based components have a `panelClass` property (or similar) that can be used to target the
+overlay pane. For example, to remove the padding from a dialog:
+
+```scss
+// Add this to your global stylesheet after your theme setup
+.myapp-no-padding-dialog .mat-dialog-container {
+  padding: 0;
+}
+```
+
+```ts
+this.dialog.open(MyDialogComponent, {panelClass: 'myapp-no-padding-dialog'})
+```
+
+Since you are adding the styles to your global stylesheet, it is good practice to scope
+them appropriately. Try prefixing your selector with your app name or "custom". Also note that
+the `mat-dialog-container`'s padding is added by default via a selector with specificity of 1. The
+customizing styles have a specificity of 2, so they will always take precedence.
+
+### Styling other components
+
+If your component has view encapsulation turned on (default), your component styles will only
+affect the top level children in your template. HTML elements belonging to child components cannot
+be targeted by your component styles unless you do one of the following:
+
+- Add the overriding style to you global stylesheet. Scope the selectors so that it only affects
+the specific elements you need it to.
+- Turn view encapsulation off on your component. If you do this, be sure to scope your styles
+appropriately, or else you may end up incidentally targeting other components elswhere in your
+application.
+- Use a deprecated shadow-piercing descendant combinator to force styles to apply to all the child
+elements. Read more about this deprecated solution in the
+[Angular documentation](https://angular.io/guide/component-styles#deprecated-deep--and-ng-deep).