It's actually not possible to change the appearance of the login box that appears when a user tries to access a page protected by Website Access Manager. This is because the program is powered by .htaccess files, which protect pages from a browser level, not through a login page. Although .htaccess is far more secure than other available methods of protection, it does mean you must sacrifice some control over design.
Here's how the process works: Your user clicks on a link to a protected page. Their web browser tries to take them there, but it encounters an .htaccess file that says, "Hey, this is a members-only area. Show me your credentials." The browser then asks your user for their login information using its built-in login box.
Since the login box is a part of the browser, you don’t have any control over how it looks. In addition, each browser's login box is unique. However, you can use Website Access Manager to change the text that appears on the login box. Go to Tools > Server Configuration, and then type the desired message into the Realm Name field.