On macOS Cocoa, setBackground/setForeground applied to a widget that is
currently hidden is not retained once the widget is realized. The "Copy
Settings" section of ChooseWorkspaceWithSettingsDialog is created with
setVisible(false), so the theme-engine colors on its check buttons are
dropped and the buttons paint with the default light system colors when
the user expands the section on a dark theme.

Re-apply the parent background/foreground to the check buttons when the
section first becomes visible. Guarded with Util.isMac() since GTK and
Windows retain the colors across realize and do not need this workaround.

Fixes https://github.com/eclipse-platform/eclipse.platform.ui/issues/3897