You can add HTML code to the form using the "Free Text(HTML)" field under the "Power Tools" toolbox on the left side.
But you cannot change the CSS of the form. That's why we always provided full form source code and allow you to edit the form source code yourself. Here is how you can accomplish it:
1. Create your form, add all the questions etc.
2. Click on "Setup & Share" toolbar tab
3. Click on "Share Form"
4. Click on "Advanced Options"
5. Click on "Full Source Code"
6. Copy the full form source code to your favorite HTML editor
7. Make your CSS changes on it and then post it on your web site.