By using a theme, you can change your website layout, without interfering with the content.
Now with themes, you can pick a layout in the style/industry you are working in, and get your website design up and running much quicker.
Be aware that some themes are a one off cost, with limited support.
The best themes are kept up to date for the latest hacking and bug fixs.
They normally cost about $100 NZD per year, but you are kept up to date.
Cheaper themes may need to be replaced after a year or 2.
You cannot keep selling the same newspaper with the same image and title on it.
You will need to freshen up your website at some point. Be prepared.
Theme Forest at themeforest.net is a good place to start looking for credible themes for your website platform.