Problems customizing.

Hello guys. I'm new to Elgg so i am still pretty confused about things. Basically, I have already built a website, I made login systems,profiles etc but it started to go wrong and I decided to try and give Elgg a go. Since I have already designed my website, is there anyway to add elgg to my websites design? or do I have to use an already made one?

I need my website to look the way it does look and I love the way Elgg operates so I would love them together.

Sorry if this question gets asked a lot. Thanks for your time!