First, please provide us a link to your wordpress.COM blog. And consider linking it to your user name as explained in the sticky, 8 Things, at the top of the forums.
Assuming that you are hosted here, you should spend a little time searching the forums and the FAQs. The CSS upgrade allows you to modify the themes in your dashboard, not to download and install a wordpress theme. Almost all of the themes available on the internet are for the self-hosted wordpress.ORG blogs and cannot be used here. We cannot upload themes, we cannot access the underlying files. CSS allows you to change the way a theme looks, not how it works.
You can try to recreate a theme you have seen elsewhere by using CSS on the Sandbox theme, but that is the closest you are going to get.
And be aware that there is no official support for CSS and only a few volunteers in the forums are well versed in it. That's why there is a try-before-you-buy preview option. It allows you to see what you blog would look like BEFORE you spend the money. There are some CSS resources at the top of the CSS forum.