In my previous post was shown that it's quite easy to create the “Operation in progress” page, but when you will notice this page is shown with the default microsoft theme. In order to use the theme that is used by the MOSS server…
string style = @" ";
SPLongOperation op = new SPLongOperation(this);
op.LeadingHTML = style + "[Enter your message here]";
currentWeb.ThemeCssUrl gets the location of the CSS file that is used.