I kind of wish that the OS provided to the browser the following pieces of theme information...

    1. is dark mode
    2. primary background color
    3. primary forground (text) color
    4. primary accent color
    5. secondary accent color
    6. warning accent color
    7. danger accent color

Even material or fluent are similar enough that applications using either tools with the above values for an overall theme would be easy enough to apply throughout an application.