This is just my personal perspective, but I would not consider that acceptable. If all you’re doing is changing a font size and color, then you didn’t build it. Sure, if you like a template and want to use it as a portfolio that you share with others go ahead and do so. I don’t consider that to be completing the given assignment though.
“Am I cheating” is a good question, and it’s one that many campers have already asked on the forums. Please see this topic for an in-depth discussion, and a quick search will bring up a wealth of other topics from your peers. Please feel free to contribute to any of those.