When the user's preferred theme is dark, apply dark styling to the
workspace selection dialog before the workbench and its ThemeEngine
start up. This ensures the dialog matches the IDE theme even at
first launch.

Styling is applied in two passes:
- A display-level SWT.Show listener as an early pass when the shell
  first appears.
- In createContents() after all dialog controls are constructed, to
  catch dynamically created widgets not yet present at SWT.Show time.

Color objects use the modern display-free Color(r, g, b) constructor
rather than the deprecated Color(Display, r, g, b) form.