Changing style of overlay container

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' })

Read this official documentation for more information.

---

ORIGINAL ANSWER

Use ::ng-deep in your component.css to override the default styles.

::ng-deep .cdk-overlay-container {
    /* Do you changes here */
    position: fixed; 
    z-index: 1000;
}

If you want to change the styling of mat-dialogue-container adding a panel class and giving style is enough, but in case if you want to change the styling of cdk-overlay-container then adding a backdropClass will help

const dialogRef = this.dialog.open(PopupComponent, {
  backdropClass: 'popupBackdropClass',
  panelClass: 'custom-dialog-container',
  data: { data: data }
});

in css add

.popupBackdropClass {
   background-color:yellow
 }