I'd be pretty over the moon if the browser supported the following preferences... especially given the number of electron or otherwise browser embedded UI options..

    system.font.primary
    system.font.header
    system.font.monospace
    system.colors.foreground
    system.colors.background
    system.colors.header
    system.colors.highlight
    system.colors.accent

It might be reasonable to have more than this, and the accent and highlight color may or may not be the same color... but it would go a long way towards matching the system defaults, with appropriate css variables injected as well.