Try to start reducing Diabook CSS

I've reduced redundant CSS in the base diabook and the diabook-aerith themes by making the -wide variants inherit from the non-wide variants. There should be very little, if any difference in the theme.

Further improvements could be made by making the -network and -profile base css files inherit from the main style.css file for each theme. Then the base style.css file for each theme could be made to inherit from the base style.css. It's a lot of work.