Yes, of course! Our themes and plugins are open source. In most cases, I have linked to the source code in Github, where you can for the project and produce your own variants. This is, after all, what I have done. My themes are based on my Qohelet base theme. However, this in turn, is based on the Quark theme, which is, in turn, based on the _S (underscores) theme. Several other open source technologies have been added by the author of Quark, and by me. So, clearly, we are happy for you to continue this process. The usual open source restridtions apply – whatever improvements you make, please post them to the community, so that we can all benefit. My own favorite method of working with the coding community is through Github, but you are free to use whatever system you please.