Licence, is theme modify and resell legal?

Hello!
i want to start web design as freelancer. I can install and use most CMS systems. I prefer systems which have css themes so i can easy modify to my needs.
Its quite hard to code a responsive theme for cross browser/device, or stuff like
"Coded with validXHTML, CSS and tested for WCAG Priority 2 Conformance."

For now i would like to use solid pre-made themes to build on-top of them.
The problem is the deep licence jungle.
For Wordpress i found this very good framework theme called Underscores: http://underscores.me/ .
Its a bit raw to start.
Can some one suggest themes or licences wich are free to modify and resell them in my webdesign service?
I dont mind the CMS, also non-CMS variants would be interessting.

For my own promotional website i have found this free GPL theme for wordpress: http://www.simplywp.net/boosan-theme.html (the cost version of the same theme has the 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 your answers!