Try editing the StyleVar, then reverting it. If I create a new theme (we created 8 recently), the stylevar is applied correctly.
My guess is that this is a bug from the 2.3.0 upgrader where the stylevar might not be present in memory right if it is the first upgrade (or fresh install) made since the stylevar was added.
I added some code to try to handle this for the next build, but users who already have a post-2.3.0 version will have to modify the stylevar themselves (or save the CSS again in the next build).