In the current version (3.1.1), there is a way to use straight HTML for the entire layout, but it's sort of a hack, isn't very efficient (any changes to any token inserted into the layout cause the entire screen to re-render) and you can't reference stylesheets. In the upcoming version 3.2, there is a new screen type called "HTML", which is an HTML page (you can specify the HTML or point it to an HTML file). It does allow referencing stylesheets. It does not support any scripting.
3.2 is in beta. You're welcome to check it out in the Beta Testing forum.