How do I add .CS functionality to XAML classes built in Expression Design?

I have a Silverlight project handed to me by designers that does not seem to have much functionalty to it yet. One of the first things I have noticed is that there does not seem to be any corresponding .cs files that match up with what appears to be children of the "LayoutRoot". I mean, as it appears in Expression Blend, these are child nodes in the "Objects and Timeline Tab" under "LayoutRoot".

When viewed in Visual Studio 8, they appear to be <Grid> children nodes of <Grid x:Name="LayoutRoot" Background="Black"> of the Page.xaml class. Shouldn't the first step be that I generate some .cs files or class to handle the functionality of these grids? I did a search on the name of the first child grid and I did not get any results in any of the existing .cs files. How do I generate .cs files?