## Description

This is an ask from some embed clients. This change is narrowly scoped
to just specific UI elements; I think it's best to keep customization
points limited to concrete use cases, to avoid complexity.

I made new CSS variables for the specific UI elements, to allow
customizing them separately. (There are plausible colors for the
button that look bad when used as the link text color.) Another reason
not to make `color-action-primary` (and other color aliases) into
customization points is to give us the freedom to use them on other
components (or _stop_ using them on existing components) without
worrying about breaking a supposedly-stable public interface.

The submit button and links are the most obvious purple-500
components, but note that there are still some others: the triangle in
the dropdown button, and the icon in the project selector. If needed,
we can allow customizing those (or maybe just make them
non-customizable and go back to grayscale); we'll see what embed
clients say.

### Followup

Once this is landed, document the customizable variables in the readme
and in the embed docs on Zuplo.

## Test Plan

Without overriding any variables, look at the form and results in all
widths; make sure it looks exactly the same as before.

Add some CSS in index.html:

```css
rewiring-america-state-calculator {
  --color-background-button: 0 255 0;
  --color-text-button: 255 255 0;
  --color-text-link: 255 0 255;
}
```

Make sure the submit button is green with yellow text, and the links
(the two in the form, plus the "learn more" links in the incentive
cards, on narrow and wide layouts) are magenta.