Related #34006

We're now currently shipping CSS bundled via the JavaScript packages that are enqueued by the global stylesheet as well. We need to consolidate this for all the use cases we have: themes with and without theme.json whether or not they declare a custom palette and gradients.

This was tried at #33924 and had to be reverted at #34108 The PR was reverted because themes that don't have a theme.json and don't provide a color palette either still need the CSS that is currently provided by the block-library. There was an attempt to fix that at #34067 but it turned out to be more complex than anticipated, so we bailed from that approach to look for a simpler alternative.