CSS styling for AuthUI

**Problem**

Provide more customization via CSS beyond the settings in the portal.

**Appetite**

2 weeks

**Solution**

- The feature is offered to users in Enterprise plan only
- In UI Settings, provide a text area for editing the css
- it will offers linting and colors for readability
- the css is embedded in the HTML served from the server in every AuthUI page
- Every stylizable elements are documented so the user can override the design easily, e.g.
- `background`
- `primary-button`
- `heading`

**Rabbit holes**

> Details about the solution worth calling out to avoid problems

**No-goes**

> Functionality or use cases we intentionally aren’t covering to fit the appetite

Start by locating the AuthUI pages and UI Settings area, then trace how page HTML is served. Done means Enterprise users can edit CSS with linting and readable colors, the CSS is embedded in every AuthUI page, and stylizable elements such as background, primary-button, and heading are documented.

Written by the indexing model from the issue text.