Code changes are at your own risk, and once they are made, we can not guarantee functionality of your Portfolio site. If you are not comfortable making code changes on your own, we suggest having a designer or web developer help you along the way.
Here is a video tutorial of some of the basics you can try.
If you'd ever like to erase your changes and go back to the default code, go to the Themes page and install your theme again, selecting the "Start Fresh" option, when prompted.
Here are some more resources to help:
HTML & CSS Help