Material Web's Typography defaults compiled from @material/web's Typescale and Typeface Sass Partials based on Material Design 3.
Based on their suggestions at https://material-web.dev/theming/typography/ I've added these:
--my-brand-font: 'Open Sans';
--my-headline-font: 'Montserrat';
--my-title-font: 'Montserrat';
--my-plain-font: system-ui;
--md-ref-typeface-brand: var(--my-brand-font);
--md-ref-typeface-plain: var(--my-plain-font);
--md-sys-typescale-body-font: var(--my-plain-font);
--md-sys-typescale-display-font: var(--my-brand-typeface);
--md-sys-typescale-headline-font: var(--my-headline-font);
--my-sys-typescale-label-font: var(--my-plain-font);
--md-sys-typescale-title-font: var(--my-title-font);
and changed all the default fonts of the scales to point to the --md-sys-typescale-<scale>-font
variable.
I don't plan to maintain this one past the point that they implement it beyond Sass mixins so use this as a jumping off point and go ham!