If you wish to submit your creation to the free theme repository on WordPress.org, it must be 100% GPL compliant, including CSS and image files. Because the freedoms spelled out in the GPL are at the heart of WordPress, we encourage developers to distribute their themes with a 100% GPL-compatible license.
So theme on official WordPress theme directory are 100% GPL?
Then as I know I can sell the themes after modifing. Am I correct?