A beginner’s question about theme/design

Hi,
I’m NEW to blogging (in fact, I’m just starting to consider getting started!), so I don’t even know which keywords to search in the forum to answer my question.

Now, I simply want to know: how complicated is it to incorporate the “blogging technology” into an existing html page? Do I absolutely have to use of the themes you offer?

For example, I know this blog uses your software: http://www.morningmantra.com/blog/ and it doesn’t seem to be using of your themes.

Thank you so much for your help.