Incorporate additional colour schemes – preparatory work

By default, our documentation is displayed in "light mode", and uses the official/approved branding colours.

We want to be able to offer multiple colour schemes to people reading the documentation (for example, dark mode, dyslexia-friendly, high contrast). I have the basic colours defined for these, but before the basic "themes" can be decided a few things need to be put in place:

- there needs to be a way in the front end for users to select their colour-scheme of choice. It would be nice if this could be a persistent setting. (for example, if someone sets their machine to use the online documentation instead of the offline F1 help accessed from a Session, they shouldn't have to change the setting every time they press F1).
- all colours need to be assigned variables. Not just background and text, but scroll bars, footer text, icons, admonitions, colours within SVG images, etc. Once the full list of variables is in place, the additional themes can be introduced.