[WelcomeUX] Why is the editor walkthrough background color fixed?

[Tyriar]

Testing #18091

On the red theme is should be a red background:

This could make the text look awkward/inconsistent on some themes (which could easily be set on welcome page before going to walkthrough).

[Tyriar]

On a related note, shouldn't the preview be the regular workbench color so that proper contrast and colors can be ensured? The same applies to the welcome page:

The blue links may also clash with the theme background.

[chrmarti]

Using CSS for the embedded editor's background. That limits us to one color per base theme. We could instead compute one from the theme's background color.