Core CSS variables

[hobbes7878]

Contingent on #75, but we want to come up with a core set of CSS variables that drive basic colours and typography on our pages.

Using CSS variables will make it easier to make changes to the overall page design in one place and see them reflected across all our core components. It also opens up the door to potentially allowing clients to customise basic brand styles in our interactive embeds (with a dynamically inserted stylesheet redefining those variables...). Brave new world stuff....

This issue is to chat through some ideas of what that core set of variables should be, starting with what we need to generally flip a page from light to dark mode, ie background and typography colours.