Exposition-wide CSS

Currently, it is possible to set CSS for the page; however, in a multi-page exposition, the styles must then be copied across to every single page of the exposition. It would be useful to be able to set CSS for all of the exposition. I assume this would have to be accessed either from the Options menu, or when listing pages on the right in the edit view, a new row at the top, before the list of pages within the exposition.

In terms of cascading, page CSS should override exposition-wide CSS.