Get rid of white space around Angular Material modal dialog

Updated Answer

From the official documentation:

Styling overlay components

Overlay-based components have a panelClass property (or similar) that can be used to target the overlay pane.

You can override the default dialog container styles by adding a css class in your global styles.css. For example:

.custom-dialog-container .mat-dialog-container {
    /* add your styles */
}

After that, you'll need to providies you css class as a panelClass parameter to your dialog:

this.dialog.open(MyDialogComponent, { panelClass: 'custom-dialog-container' })

Link to StackBlitz Demo. Read this official documentation for more information.

Original Answer

Use ViewEncapsulation to override default styles with your styles.

In the component which opens the dialog, do the following changes:

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

and in that component's css file, add the following class:

.mat-dialog-container {
    padding: 0px !important;
}

Here is a link to Plunker Demo.

I made a comment on the chosen answer, but I'd like to clarify in a full answer. If you add dialog styles to your component and set ViewEncapsulation to None, those styles will globally affect your whole app, and any future dialogs will open without padding.

Instead opt for utilizing the dialog's panelClass property:

component.ts

this.dialog.open(MyDialogComponent, {
  panelClass: 'app-full-bleed-dialog', 
  data: ...
});

global stylesheet

.app-full-bleed-dialog .mat-dialog-container {
  padding: 0;
}

That way, you can still keep encapsulation on your dialog component styles, and you can selectively reuse your app-full-bleed-dialog class as needed.

To read more about customizing Material components, check out this guide.