Remove all default css and make Raleigh a pure fallback theme
It was problematic to maintain Raleigh going forward, as any changes in it affected all themes. Also, its more robust if each theme is a full standalone css rather than relying on an inherited css base. So, this changes Raleigh to a standalone theme that we can tweak without accidentally breaking other themes, and makes the default theme empty. In fact, we don't even add the default provider anymore as its always empty.