Well, it is definitely not a stylesheet editor for your Vista Windows OS. In fact it has nothing to do with Vista OS, but only with CSS.
It is a free product that should provide you the ability to edit live the CSS of your site on both IE and Firefox. That’s sounds cool! Editing live sites on Firefox has been available for a long time through the usage of Web Developer extension or the great Firebug plugin. But nothing similar, not even close, was available for IE. Now having this product will hopefully make our web dev life a bit easier, and having it for free is even great. Congrats to the Litmus guys!
You can find the product here.