Move `styles.CSS` to `elem.CSS`

[chasefleming]

Problem

Because the Node interface has been added updated this breaks.

Suggested Solution

There is probably no need for CSS to be in styles as this creates a circular dependency. We could either just use elem.Text to set it in the style tag or add an additional function for elem.CSS.

styleTag := elem.Style(nil, elem.CSS(cssContent))