Trying to understand theme copyright issues

I am trying to understand copyright material. I want to create some WP themes for sale. My question is can you take a theme and redesign it if you totally change the look and feel of the whole theme. I would never try to steal someone’s work or take credit for other’s ideas. Is the theme structure copyrighted or just the pictures and color set up?

I know there are some themes who allow you to make any change you want. Any help would be appreciated.

Thanks!