Licence, is modify and resell legal?

Hello!
If i want to customize a theme and resell it via webdesign charge, what licences are legal in that case? This licence jungle is quite deep.

I want to start selling simple webdesign as freelancer. For my own promotion website i have found this free theme: https://www.simplywp.net/boosan-theme.html
(the cost version of it have same licence)
its legal to modify it in any way and use it comercial, but only as client, not for reselling, as the author responded to my request.
Thanks for answers!