Thanks @huochunpeng. I got the useDefaultSettings() from the Aurelia docs https://aurelia.io/docs/plugins/dialog#styling-the-dialog. I guess it was an error on my part to assume cfg.useDefaultSettings() was just another way to write config.useDefaultSettings().
And thanks for the note that .useCSS('_dialog.scss') wouldn’t work. I totally overlooked the fact that the SASS would not be compiled. Or at least that I what I am assuming after reading your comment.
I have added the dialog styles to my base styles as you suggested. Thanks!