Ran the `npx @tailwindcss/upgrade` and accepted the changes that seemed safe.
They're things like:
- `data-[closed]:translate-y-9` -> `data-closed:translate-y-8` ()swaps the square bracket syntax to a `-` modifier)
- `bg-gradient-to-*` -> `bg-linear-to-*`
- `/[*%]` -> `/*` (swap square bracket syntax for inline)
- `theme(*.*)` -> `var(--*-*)` (theme styles are exposed as variables with hyphens for dots now)
- `[background-size:*]` -> `bg-size[*]` (move the square brackets inside tag)
- `[.active_&]:` -> `in[.active]:` (new syntax for parent query)
- `!class` -> `class!` (e.g. _!overflow-visible_ to _overflow-visible!_, for [important flag](https://tailwindcss.com/docs/styling-with-utility-classes#using-the-important-flag style)
- `w-[1px]` -> `w-px` (that's a new syntax for a 1px width)
- `h-[1px]` -> `h-px` (that's a new syntax for a 1px height)
- moved `html` and `html, body` global settings in the _index.css_
Also killed off an unused `import` and blank css class.
Also picked up the two `flex-grow` -> `grow` that I missed last pass, oops.