Suppose that I want to create a commercial site for a company. As I found I can use WordPress for this purpose. As I understood I can use one of free GPL themes (http://wordpress.org/extend/themes/) too.
Can I modify free themes freely for use on my commercial site?
1. Is anything that I should leave in theme? Such as link to the first author/Wordpress or anything else?
2. Am I forced to anything? Such as putting the modified theme sources somewhere or anything else?
I've investigated in GPL licence but I still can't understand how it works with webdeveloping and skinning.