All themes state their license in the style.css file and most, including the Default theme, are licensed under the GPL. Considering what you want to do, this is the most relevant part of the GPL:
You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the source or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
In other words, if you modify a GPL theme, you may modify and redistribute it, so long as it is also free and licensed under the GPL.
Basically, if you want to sell a theme, you'll have to build it from scratch.
after some time of digging, and hopefully this is relevant not only for me:
The term free in the whole licence is referred only to the fact that the program is allowed free usage and changing, not free as in "free beer"
the quote you used means tht the program must be LICENSED free of charge, which is not something that non-free software does.
basically, when you provide someone a copy of a free software, you give him the licence too.