The dialog now displays which product or application the default theme
applies to, making it transparent that theme defaults are scoped.

Examples:
- With product: 'Current default theme for Eclipse SDK (org.eclipse.sdk.ide): Dark'
- Without product: 'Current default theme for org.eclipse.ui.ide.workbench (org.eclipse.ui.ide.workbench): Dark'
- Without either: falls back to unscoped messages