Reduce Customisability of default fonts and colours

We should provide some pre-defined 'themes' that are inline with user's accessibility needs in order to minimize the number of potential fingerprinting buckets.

This would require a few parts:

• identifying which sets of default provided 'themes' we should provide

  • it seems some set of options could be:
    • default || dark mode
    • default || high-contrast
    • default || large text

• new UX for this entire section of about:preferences:

  

/cc @henry @donuts @thorin