Once you purchase an item (theme/plugin/script), you can make any changes you want to it for your personal use; but based on the type of purchased license, you can’t resell it.
Also, the author is not required to assist you with any customizations and if your customizations “break” or “adversely” impact the item, the author is also not required to help you fix it afterwards, as you were the one who broke it. Authors are only required to provide support if the item you purchased is used in the same original scope as it was sold to you.
So, if you are willing and able to accept those limitations, nobody can stop you to make whatever changes you want to make. 