Hi Darren Ratcliffe,
The light grey stripe is part of the design of that particular skin of RadWindow that you are using.
As it is an image, it cannot be modified by the means of CSS, but you need to edit the image sprite that contains the three images that the grey stripe consists of - left, middle and center.
In order to modify the skin, you need to set it as an external one by using the EnableEmbeddedSkins="false" property of RadWindow, and registering the skin in the <head> of your webpage using the <link> tag.
The uncompiled skins are stored in the [RadControlsInstallationFolder]/Skins/Telerik/Window.Telerik.css .and [RadControlsInstallationFolder]/Skins/Telerik/Window/
the Telerik team