Skip the early dark-theme styling for Button controls in the workspace
selection dialog at startup so buttons get the native macOS dark
appearance instead, matching what the ThemeEngine applies later.

Other widgets (Shell, Composite, Label, Link, ExpandableComposite)
remain styled as before since the ThemeEngine is not yet available
during workspace selection.

On windows we still need to style the check buttons otherwise they look
white.

Fixes: https://github.com/eclipse-platform/eclipse.platform.ui/issues/3905