Clicking the site root (just above "Home" in the Page Tree) reveals the theme drop-down selector in the main edit window of the CMS. Very nice.
I find that I can actually comment out the line in /mysite/_config.php and it still works just fine.
A quick tip if you are not seeing your theme in the drop down list for the CMS; Do not add any underscores (or perhaps any special characters) to the theme name.
As soon as I changed my folder name from: "theme_name" to "themename", I could select it and it worked.