This is pretty easy, but it does need some extra lines added to your custom folder.
Perhaps this could be a config option in the future?
Thesis: Move the navbar Below the Title
You need to add the following code to the custom_functions.php file.
Just paste it at the end.
What does this do?
This little bit of code removes the standard call to the thesis_nav_menu function that builds your navbar and then adds it back in again – but this time it is built after your blog header is outputted.