Using float in your panes. But could you be more specific? What do you want to achieve?
Do you mean giving the container a fixed size instead of using the entire browser width? You could try the following:
.tundra .MxClient_mainContainer {
width: 980px;
margin: auto;
}
Be sure to read our tutorials or using the custom theme package from the tutorials.
You cannot create new stuff using CSS. It is only possible to amend the styling of existing items.