Extra CSS files are overridden by themes

Potentially a bug, perhaps just for certain use cases... but any CSS files that are included in the extra CSS setting (with system_stream_wrappers) are added to the page at the 'default' level/group (same as modules?). CSS files of the [admin] theme will be after these. But as an editor/administrator, I would expect the specific extra CSS files to override existing CSS, at least all the default-level, and probably the theme-level CSS, too.

The attached patch adds the extra CSS files at the theme level, which appears to make it come out after the theme's existing CSS, which is what I would want. Weights could be specified, but this looks to work correctly to ensure the theme's CSS is overridden, probably just because of the point of the page request at which the CSS is added.