[6.x] A couple of layout issues with code fieldtype

[helloDanuk]

Bug description

1. Light Mode: No Borders in lower section

There is only a shadow, but no border in the lower section of the field

2. Layout shifts

Text jumps up and down when changing CP color mode under the profile image

https://github.com/user-attachments/assets/d19514b2-ce68-44af-a64b-f2410390fe45

3. Changing CP color mode leads to low contrast

Take a look at a code field with light mode. When you change the CP Color mode to dark, it slighty changes the colors in the code fieldtype too. See for example the light grey "Lorem ipsum" with low contrast.

4. Fullscreen mode: Can't change syntax highlight mode

https://github.com/user-attachments/assets/96612564-4d0d-47fb-8f78-14e3cadb03f1

5. Fullscreen mode: Gap between Header and code editor

How to reproduce

Add a code fieldtype to a blueprint.

Logs

Environment

6.0.0-alpha.18

Installation

Fresh statamic/statamic site via CLI

Additional details

No response