I purchased a theme for a client and the design was well thought out and just about perfect for what I need to do. However the code base and some of the functionality was not what I expected which was surprising giving how well the design was. I checked and discovered that the theme was actually built about 5 years ago. I’m dealing with a high paying client so the theme will need some heavy improvements especially in the area of performance along with some better suited plugins. I would like to rebuild the theme using my own framework and only use the assets that was used to layout the design and styling of the theme( css,js, etc). Does this violate the Regular license in anyway?. In the future could I just purchase an html template and use the design assets to create a wordpress theme specifically for a client?
I think I’ve already found the answer in the FAQ 
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Can I license an item and then use components from it, instead of using the entire item?
Yes, to some extent. You can modify an item and so you’re allowed to delete components from it that you don’t want (still only using the item in one end product). But you are not allowed to take just one single component from an item and use it on a stand-alone basis.*