CodeAcademy is a popular option for expanding upon your CSS skills. The one downfall is that some students complete the work, but aren't exactly sure why it worked!!! I like to think that having completed this course, you would be able to learn some new material and grasp some of the lessons they aren't specifically explaining. The new "Make a Website" skill set is a great place to practice what you learned. It also goes a little bit into the responsive design idea.