In this article, we'll show you how to customize the logon, language selection, and logoff pages, and how to create a theme by using a custom header. If you want to create an Outlook Web Access theme that has more complex changes for your organization, begin by studying how cascading style sheets (.css files) work. After you have developed a good understanding of .css files, change a few elements at a time and test your changes to make sure that the result is what you expect.