The newsletter widget with a Razor theme may not work... at least one other person has brought this up. Three possible solutions I can think of.
#1 - Code changes for the Razor host page that would attempt to inject the ASP.NET validation scripts into the page.
#3 - Create a Razor version of the Newsletter widget. Or modify the existing Newsletter widget so it is able to dynamically choose between the non-Razor version and the Razor version based on whether the theme is a Razor theme.