Angular custom style to mat-dialog

This will definitely work:

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

You should use panelClass on the component at the same time as ::ng-deep on the css.

openErrorDialog(errore: string): void{
let dialogRef = this.dialog.open(ErrorDialog, {
    width: '80%',
    data: { error: errore }
    panelClass: 'custom-modalbox'
});
}

in css:

::ng-deep .custom-modalbox {
mat-dialog-container {
    padding: 0;
}
}

I just change this, it works perfectly:

.custom-modalbox >  mat-dialog-container {
        padding: 0px;
}

Here there is a working example: https://stackblitz.com/edit/custom-dialog?file=src/styles.css

You can pass a custom panelClass in your matDialogConfig Object (doc here)

so

openErrorDialog(errore: string): void{
    let dialogRef = this.dialog.open(ErrorDialog, {
        width: '80%',
        data: { error: errore }
        panelClass: 'custom-modalbox'
    });
}

And in your custom panelClass you can override the padding :

.custom-modalbox {
    mat-dialog-container {
        padding: 0;
    }
}

But your .custom-modalbox should be global scoped to be applied (not defined in the component styles )