Workaround: When my next item in the Service requires a different Theme, then I'm forced to right-click on the new Theme and set it as Global Default Theme before I go Live with the new Service item.
It was nice when we could select the Theme for each item in a Service and save the Service with multiple Themes. Then we could open the Service and it would use the previously saved Theme that it had been connected to.
However, I like the new 2.9.4 and it's new functionality. This workaround is ok with me for now.