The error indicates that it's looking in the wrong place for your themeDefault.aspx page. It should be looking in C:\BegASPNet2\Chapter05\themeDefault.aspx. Can you confirm a couple things for me please?
1) Where your copy of themeDefault.aspx is located on your hard drive?
2) What you put in for the @Page directive for themeDefault.aspx (which is where it tells it took for the theme)?
3) Where your copy of themedmaster.master is located on your hard drive?
The other day I managed to break a site I was working on by accidentally getting ./ mixed up with ../ ! My guess is something similar is happening here, although there's probably a chance it's going to turn out to be something way more complex. I need to confirm that it isn't something simple first, before we start digging deeper.