Test theming in the welcome page

[aeschli]

Testing #25798

Complexity 2

• AnyOS @aeschli (linux)
• AnyOS @seanmcbreen

(Workbench colors can be tested with the "workbench.colorCustomizations" setting.)

• Check the Welcome Page, Interactive Playground and Interface Overview look decent with all built-in themes.
• Check the Welcome Page supports customizing these colors:
  • welcomePage.caption
  • welcomePage.detail
  • welcomePage.quickLinkBackground
  • welcomePage.quickLinkHoverBackground
• Check the Interactive Playground supports customizing these colors:
  • walkThrough.embeddedEditorBackground
• Check the Interface Overlay support customizing these colors:
  • welcomeOverlay.foreground
  • welcomeOverlay.background