It’s slightly complicated. And the following is only as far as I know, so don’t take it too seriously.
Yes, in general, you’re allowed sell themes, but since you’ve based your theme on Kubrick (you said it yourself), and Kubrick is released with the clause:
The Kubrick template is free and as open source, allowing you to make whatever changes you see fit, and even release the results, as long as you release it under the same terms as you received it.
you have to make sure you abide by that. Which in reality might means you can probably sell it, but you have to give the person you’re selling it to the same rights (rights to use, modify and distribute), so (s)he can give it away free of charge. So why would another customer have to come to you?
Anyway, it gets more complicated, since you own the “art”, you could sell that, in your own terms. But since in effect that’s only your header picture (the rest are just modifications of colour values in the “source”), I am not sure how much you can get for a picture of a leaf (even though it is pleasing to the eye).
Long story short, it’s theoretically allowed. But probably not practical in this case. If you were to start from scratch however, all that code will be yours, so you can distribute it however you see fit. But again, there is a slight grey area because you’re depending on bits of WP for functionality, and WP is GPL.
I think I’ve confused you totally.
Ok, final summary. The art is theoretically yours, and can be sold, under whatever terms you want. As long as it’s not derived from something that imposes rules on your distribution.
