Design tokens — refinement to disabled and border colour tokens

We are intending to make some changes to the design tokens schema to:

- [ ] Add a new `light` border colour
- [ ] Refine the colour tokens used to indicate disabled/inactive text and UI elements to make them more legible
- [ ] Add a hover colour for hyperlinks (per request from user)
- [ ] Add a new 0.75rem token (between `medium` and `small`) for padding and margin

These changes should not be breaking, but we will investigate and confirm whether this will be a major or minor release.