Use more effective CSS statements

Some diazo themes use general styling, such as:
```css
#content ul {
    list-style-type: disc;
    margin-left: 2em;
}
```

This breaks the simplelayout menu, since it is prefixed with `#content`.
In order to make the simplelayout menu stylings stronger, I've prefixed the most important statements in `simplelayout.css` with `#content` too, so that they will be used even when a later theme css styles `#content ul`.

I've also removed the use of `#portal-columns`, using `#content` instead, since the `#portal-columns` may not be there in a diazo based theme. It is more common to use `#content`.