Modifying WoltLab Blue Sunrise

Hello,

Just a question of out curiousity. Since I'm building a site from scratch I got a fresh install of wbb 3.1 with the wbb 2.1 portal addon and the custom pages plugin to work with. The only thing I'm missing now is a custom style/layout and since I'm handy with Photoshop and some of my friends are designers I am eager to start working on a custom style.
However, since I or my friends cannot code I got to use a style as a basis and the WoltLab Blue Sunrise style (commercial license) works really great since it loads really quick and doesn't use a lot of images. It also works very nice with the wbb 2.1 portal addon and I like the way it's structured. So my question is the following:

Is it allowed to modify the WoltLab Blue Sunrise style (think of it as replacing most images that give the style it's look) and use it on my own wbb 3.1 board? All copyrights will stay intact, it will only be used on my own website and only on my wbb 3.1 board and won't be shared with anyone.

Thanks in advance!

p.s. Just to be clear, I will be modying the style myself in Photoshop and won't share or distribute any part of it.