* Guides feedback fixes

@claudioduarte in https://github.com/rails/rails/discussions/51415

- [x] Make alt, shorter SVG line for mobile, add responsive style
- [x] Add side padding on footer (goes to edge on mobile, does not align on desktop)
- [x] Shrink overall height of subCol so it doesn’t overlap footer

@ivanjuric in https://github.com/rails/rails/discussions/51398

- [x] Flip inverted dark modes on “more Ruby on Rails” nav bar

DHH/Core:

- [x] Darker dark mode (Github style)
- [x] Double box on WIP call out (index.html. editable?)
- [x] NavBar bottom issue clipping on some guides (pad the content?)
- [x] Rounded corners/better appearance on select form elements in header
- [x] Black border on bottom of interstitials

* Additional UI cleanup

- [x] tightening up white space, adding Rails brand HR after intro
- [x] base fontsize to 16px from 18px;
- [x] fixing some darker dark mode issues in the last commit

* Lightening syntax highlight colors