skin customization in app

I have followed the doc pages for customizing a selected skin (flat.css) by accessing the webix.skin.$active.backcolor property (for example) and have set it to a diffierent color (red instead of white). Then I do the normal webix.ui(…) setup but the displayed app in chrome stays white. I use webix 8.0.1 and a direct change of the skin via html tags works. Is the change as documented not working?