fix(www): Showcase modal prev/next on small screens (#18988)
fix(www): Showcase modal prev/next on small screens (#18988)

* rotates prev/next link text at medium bp, rotates arrows up to medium bp, adjusts prev/next img margins

* Use `theme-ui` and `space` tokens

space[6] = 24px
https://theme-ui.com/sx-prop#margin-and-padding

* Consolidate container styles, remove dead code

* Fix prev/next link color for dark mode

* Remove fixed width for prev/next links on mobile

… and make them 100% wide on mobile.

- ditch a few more obsolete styles and divs
- move outer margins to `<Link>` and adjust `transform` and `top` values
- remove margin from gatsby-image via `imgStyle`

gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby