Unable to set default theme at space level when another theme is set as site-wide theme

From this thread on the Confluence Forum:

I am testing an upgrade from 2.9 to 3.0.1, and can't seem to use the default theme in spaces unless it is set as the site-wide theme in 3.0.1.
Previously, in 2.9, I had set left navigation as the site-wide default. For any space, I could simply select the original default theme as one of the options. In 3.0.1, I am only given 3 options: use default (which is still set to the site-wide theme of left navigation), sharepoint decorators, clickr, or left navigation. It seems the only way I can use the default theme is to set it as the site-wide theme.
This is problematic, however, because it would mean that we would have to go to each space that was previously using the left navigation theme through site wide default, and manually change it to left navigation. We have many spaces set up for groups in our facility, and this would be a lot of effort.
Did I miss something, or is there another way to do this?

Thanks