Streamline the common css across themes.

[joomlart]

At the moment if you have a number of themes created for the template the size of the zip becomes very large because we end up packing a new version of the same css file in each theme folder.

I know why this was doen to simplify the process of finding the css but I think that it would be better to have this structure:

css/
css/all common css files here
css/theme/mytheme/only css related to the theme.css