Built-In Stylesheets
If your Built-in stylesheet versions indicator says anything except "Current", your stylesheets will need to be updated before building the project. You will be asked if you want to allow this (the program will do it), but if you decline, your project will not be built.
If you have modified the original stylesheets (slLayout.css and slStyle.css), you may want to save them somewhere before building. This will allow you to go back and see any changes you had made for your web pages to reproduce them after the build, should your pages look too different.
As is mentioned elsewhere, it is far better to create your own stylesheet(s) to override the original specifications, so you can avoid this problem.
