Eclipse Luna M6 allows you to include other CSS stylesheets. For example to use the new Eclipse dark theme in your RCP application, do the following:
In your RCP application created a /css/default.css file with only one instruction
Add org.eclipse.ui.themes to your RCP product and add the applicationCSS property pointing to platform:/plugin/test/css/default.css in your product extension point. If you start your application is is dark styled.