RH8: How do I choose a stylesheet to apply for all topics?

I have a RH project created by importing a FrameMaker doc (foo.fm). All (most, but I'm not going to worry about that) of the topics came in okay, and a stylesheet was created and added to my Project Manager: "foo.css".

Now, I have a different .css file that I want to actually use: bar.css.

I was able to add bar.css to the Project Manager (thanks to an old forum post), so now I have foo.css and bar.css.

All of my topics are currently using foo.css. I can change them all to use bar.css by selecting the entire topic list and changing the assigned style guide.

That leaves me with... how do I ensure *new* topics automatically use bar.css?

Thanks!