Hi there
I have less experience with blog templates but with most blogging sites you usually have an option to adjust the code and you should be able to paste in the code for the design you want to use. This will vary from site to site and some limit what you are able to change more than others.
A quick Google search for free blog templates will throw up a lot of sites and many of them have FAQs qith instructions for how to apply them to your blog to make it much easier for you
I hope that helps, and please come back to let us know how you got on and what you chose!