Same theme across all subdomains on WP MultiSite?

I am developing a website for a client at http://monsterdealengine.com and I've set up WPMU so that subsites are using subdomains. I deployed a clone of a website that I have working elsewhere that will be the base of the website I'll be building on this domain. I then set up a test subsite of http://pgp.monsterdealengine.com thinking that the same theme used on the main site would simply propagate over to the subsite; however, it didn't.

I went into the dashboard of the subsite and chose the cloned theme and "some" of the things on the main site show up on the subsite, but not all of the CSS is carried over. I was under the impression that without installing a plugin to allow each subsite to have a different theme, that all subsites would have the same theme. I understand that I can modify certain things (such as images and logos that are used in the subsites), but for the most part they would have the same look and feel. Am I incorrect in that understanding?

If so, then how can I modify the CSS for subsites so that they are similar (but each with their own differences? I actually thought I had studied MultiSite enough that I understood it to be what I needed, but now I'm thinking that I either misunderstood, I installed and set it up wrong, or I've not installed some plugin that would get me where I need to be.

Any help would be greatly appreciated. I've ready through the tutorials and FAQ of this site since joining earlier today and I'm no further along to understanding why it doesn't work like I thought.

Regards.

James Dunn
Athens, GA USA