There are two ways to embed Clara.io scenes on web pages. The first is via the iFrame embedding method. In this method, you can not change the CSS. But the second method is via a div frame. That way you can change the CSS, but it is tricky. Here is how to do the div-frame style embedding:
Also the play button only shows up if you have an animation. Just remove all keyframes and it will not appear.
Maybe you can explain how you want to change it and we can see if we can help?