Not really, not unusual to see themes being made for good software. I've been designing since 1997 and have designed mass template themes for many useful open source and commercial projects.
When I find an open source project that I like and will use for my clients I always feel the need to support it in some way, whether it be helping with support forums or theme design, same as I've done with Website Baker for years.
OpenCart is an excellent product with a bright future, I intend to offer it to my clients once it's at a stable version. There's plenty of room for anyone capable to help support the users and Author with contributions.
I'd believe your on board with the same philosophy.
I will eventually offer Custom themes for sale as you are, or provide custom work, but for now I want to become more in depth with the software before I do.
Look forward to seeing more of your nice themes for the users, keep them coming
Nice to meet you and if I can be of help feel free to contact me
JC