You can change the
LookAndFeel of the standalone window by calling
LookAndFeel::setDefaultLookAndFeel somewhere in your plugin code to your desired
LookAndFeel subclass. Just be aware that it will also set the
LookAndFeel of the containing plugin if you don’t set the contained plugin’s
LookAndFeel to something else!
As for changing the actual layout and controls of the standalone window, AFAIK there’s no way to do that without going in and changing JUCE code…