Match window border color to app theme rather than OS theme (#958)

* MacOS: match window chrome to app light/dark mode when app setting is different from OS

* match window border to app theme on Windows

ui/mainwindow.go

@@ -557,6 +557,9 @@ func (m *MainWindow) addShortcuts() {
 func (m *MainWindow) showSettingsDialog() {
 	m.Controller.ShowSettingsDialog(func() {
 		fyne.CurrentApp().Settings().SetTheme(m.theme)
+		for _, w := range fyne.CurrentApp().Driver().AllWindows() {
+			controller.SetWindowThemeMode(w, m.theme.AppearanceMode())
+		}
 	}, m.theme.ListThemeFiles())
 }