How to make a widget sticky with Upfront (Spirit)

On my test-project http://oelziehen.net/ I have a navigation widget, and I want to make it sticky (= alway visible when page is scrolled). Its a normal text-widget within a sidebar-region. (http://screencast.com/t/JBWloPA9u)

Could anyone help me with the CSS to achieve this? That would be great! I also granted access if needed.